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Translated with the Author's permission, from the German r 



Fellow of University College, London. 



and H. B. BONNER, 

1 & 2 TOOK'S COURT, E.C. 

\Off Chancery Lane and Holborn 







Loyal Fellow-Traveller in Sicily and Naples, 



Faithful Comrade in the Fight for Truth,- 










I. On The Darwinian Theory. 

Delivered at the first public sitting of the thirty-eighth meeting of 
German Naturalists and Physicians. Stettin, September 19th, 1863. 

II. On the Origin op the Human Race. 

Delivered to a private circle of friends. Jena, October, 1865. 

III. On the Pedigree op Man. 

As the last. November, 1865. 

IV. On the Division op Labor in the Life op Nature and of 


Delivered in the Hall of the Berlin Artisans' Association. Decem- 
ber 17th, 1868. 

V. On Cell-Souls and Soul-Cells. 

Delivered in the Concordia, Vienna. March 22nd, 1878. 



VI. On the Progress and Work op Zoology. 

Delivered on entering the Philosophical Faculty at Jena. January 
12th, 1869. 

VII. On the Development op Life-Particles and the Peri- 

Delivered to the Medico-Scientific Society of Jena. November 19th, 

VIII. On the Proofs op Evolution. 

Delivered to the same Society. March 3rd, 1876. 

IX. On the Present Position of Evolution in Relation to 

Delivered at the first public sitting of the fiftieth meeting of German 
Naturalists and Physicians. Munich, September 18th, 1877. 

X. On the Origin and the Development of the Sense-Organs 
Delivered to the " Scientific Club," at Vienna. March 25th, 1878. 



central idea that all the lectures in the following 
-A- volume have in common is the monistic principle 
of the unity of development, of mechanical causality 
throughout Nature. It is that same principle, the 
most important in modern science, on which rests the 
law of the Conservation of Energy, the principle that 
Kant has already pointed out as indispensable to every 
true explanation of things, the principle to which 
Goethe has so often given admirable expression, that 
Lamarck, in 1809, laid down as the foundation of his 
Transformism, and that Darwin, in 1859, utilised in 
general Biology by his theory of Natural Selection. 
How this monistic principle is applied in different 
branches of knowledge, these lectures are intended to 
show. They are arranged so as to be easily under- 
stood by the cultured who are not especially students 
of science. As they have been delivered at different 
times, without any connexion one with another, and 
have been given under different circumstances, without 
regard one to the other, repetitions have been inevit- 

Although to remove these repetitions would have 
been of value to the collection of lectures as a whole, 
this would have been impossible without completely 
reconstructing the individual lectures. Further, much 
of the interest that has given rise to the wish among 



my friends for the publication of this collection lies 
actually in the exact form and method of treatment 
that the study of development, itself developing, has 
received at different times in my earlier discourses. I 
have therefore considered it best to publish the lectures 
unaltered in their original form. In certain places I 
have struck out palpable errors and added some new 
annotations. Further, I have naturally chosen for 
publication the latest, most improved edition of those 
lectures that have passed through several editions. He 
who wishes to instruct himself more fully upon the 
questions treated here, will find further information in 
Darwin's Works, in " Cosmos " (a publication dealing 
with the study of the universe in the light of evolu- 
tion), in Carus Sterne's " Growth and Decay " (a 
popular history of development in Nature, Berlin, 1876, 
Eggers), as well as in my own larger popular works, 
"The Natural History of Creation," 6th edition, and 
" Anthropogeny," 3rd edition. 

The first of the five lectures in this first part is on 
the Evolution theory of Darwin. It was delivered on 
the 19th of September, 1863, at the first open sitting of 
the Thirty-Eighth Congress of German Scientists and 
Physicians at Stettin, and until the present time has 
only been printed in the official report of the Congress. 
As the proof of this report was not sent to me, I was 
unfortunately not in a position to correct the many 
gross printer's errors therein. I was very desirous of 
an opportunity to publish this lecture, the more so as 
in it the Evolution theory of the present day was 
brought for the first time, viva voce, before an assembly 
of German scientific men. It was an experiment by 



no means easy or devoid of danger, an experiment 
also not without its results, as I have pointed out in 
the preface to the fourth edition of my " Natural 
History of Creation." 

The three following lectures (II., III., IV.) first 
appeared in the " Series of Popular Scientific Lectures" 
issued by Virchow and Holtzendorff (Parts 52 and 53 of 
the Third Series, Part 78 of the Fourth). The two 
lectures " On the Origin of the Human Race " (II.)? 
and "On the Pedigree of the Human Race" (III.), 
appeared in the first edition of 1868, in the third of 
1873. But they had been already delivered in identi- 
cal form with that they have at present in October and 
November, 1865, to a small private circle at Jena, at 
the request of my lost friend August Schleicher. This 
renowned philologist, whom premature death reft from 
science and his friends in 1868, took the keenest in- 
terest in the monistic theory of Evolution, and devoted 
himself with the greatest success to the department of 
comparative philology. In consequence of our many 
conversations thereon, and moved by the many attacks 
that my first lecture in Stettin had brought upon me, 
he sent me a letter on the Darwinistic theory and the 
science of language (Weimar, 1863). In that letter the 
reader of this volume will find an able investigation of 
Evolution from the linguistic side. Further, Schleicher 
especially urged me to follow up the anthropological 
side of the descent-theory. Hence, in 1865, the two 
lectures on the origin and the pedigree of the human 
race. These are to be considered as forerunners of my 

The fourth lecture " On the Division of Labor in the 



Life of Nature and of Man," was given December 17th, 
1868, before a mixed audience in the hall of the Berlin 
Artisans' Union, and appeared in 1869 as the Seventy- 
Eighth Part of the Virchow and Holtzendorff Series. 
A second or third edition appeared in 1873 (the title- 
page does not give the edition). The wood-cuts be- 
longing to this have been improved and increased in 
number in the present issue. 

The fifth lecture on "Cell-Souls and Soul-Cells," 
was delivered March 22nd, 1878, in the Concordia 
Gathering at Vienna, and in July of that year appeared 
in the German Review (Part 10 of the Fourth Annual 
Series), but without the wood-cuts that illustrate this 
lecture in the work now under consideration. Sepa- 
rated by an interval of ten years from the four pre- 
ceding lectures, it stands in close internal relationship 
to these, and repeats, indeed, some of their details and 
figures. On the one hand, the pregnant principle of 
the division of labor finds in the souls of the higher 
animals, and especially of man, its fullest application, 
clearing up the dark mysteries of spirit. On the other 
hand, the soul-cells have really been produced by 
division of labor from cells gifted with a common cell- 
soul. They explain, therefore, admirably the physio- 
logical process of division of labor. Of purpose, 
therefore, is reference specially made in both lectures 
to the Siphonophora, that interesting and instructive 
class of sea-animals. The extraordinary significance 
of the Siphonophora in the study of soul-life is less 
clearly recognised, but is not of less moment, than 
their great use in the thorough understanding of the 
principle of division of labor. 



May these popular lectures, thus gathered together^ 
aid in advancing the knowledge of our Evolution 
theory of to-day among those who, though not them- 
selves scientific students, are yet firmly convinced as 
to the necessity of a clear, unifying conception of the 


Jena, October 12th, 1878. 




ALTHOUGH in the lecture I am about to give I 
shall try to discuss with you the principles of 
Darwin's celebrated theory of Evolution, I do not make 
the attempt without great anxiety, nor without asking 
your forbearance for what may be lacking in its pre- 
sentation to you. For to an audience like the present, 
composed of laymen and men of science, all popular 
treatment of a theory that is scientific and far-reaching 
must present much of doubt and difficulty : it can 
never do more than satisfy a part of the listeners. Of 
modern studies, that one must be in a very special 
position, which threatens to shake to its very founda- 
tions a vast structure of human thought, that has 
boasted and yet boasts of almost universal acceptance 
through many centuries, and also touches the personal, 
scientific and social views of every individual at the 
heart's core. That the question is of an idea that 
modifies the whole conception of the universe those of 
you not yet acquainted with the tenor of the Darwinian 
account of that universe will see at once, if you listen 
to the principles thereof, as embodied in the following 
words : " All the different animals and plants living 
to-day, and all the organisms that have ever lived on 
the earth, have not been created separately, each after 
its kind, as we have been wont to believe from our 
youth, but have gradually developed in all their 
manifold forms and varieties in the course of many 
millions of years from a few, perhaps from one single 
primordial form of the very simplest nature." 

Therefore, as far as we, as human beings, are indi 
vidually concerned, we have, as we are the most highly 
organised Vertebrates, to seek for our ancient ancestors 




in ape-like mammals, beyond these in kangaroo-like 
marsupials, still further back in the so-called secondary 
period among the sauroid Reptilia, and finally, at a still 
earlier time in the primary ages, amongst the lowly 
organised Pisces. 

In the limited space of time of one short hour it is 
clearly impossible to lay before you from the store- 
house of science even the most weighty and striking 
arguments for and against this able hypothesis. To 
estimate these rightly, and to be able to apply them, 
one essential is a close study, extending over many 
years, of the structure and functions of organisms, 
their affinities and histories. If, despite these and 
many other difficulties, I try to draw you into the 
strife that has arisen as result of Darwin's theory of 
Evolution, it is because of the vast dimensions that this 
strife has already assumed. Already the whole vast 
army of zoologists and botanists, of palaeontologists and 
geologists, of physiologists and philosophers is divided 
into two widely separated parties. On the standard of 
the progressive Darwinists are the words " Evolution 
and Progress." From the camp of the conservative 
foes of Darwin sounds the cry : " Creation and Species." 
Every day the gulf widens, the two orders of thought 
become more asunder. Every day new weapons of 
offence and of defence are brought forth on all sides. 
Every day wider circles are embraced by this powerful 
movement ; far-off things are drawn into its eddy, and 
whether he will or not, the man who would fain hold 
aloof from both parties must give in his allegiance to 
the one or to the other. Already the Darwinian theory, 
at first scoffed at as a wild speculation, numbers among 
its adherents the majority of the most famous men of 
science. For example, I make mention of but three of 
its most prominent advocates in England, Huxley the 
zoologist, Hooker the botanist, Ly ell the geologist. The 
last-named is the more notable an advocate, as he was 
in earlier times an opponent of Darwin. Under these 
circumstances it seems to me that it should be the aim 
of every scientific man not to approve of that which 
will stifle and choke the whole inquiry, that which is the 
design of the esoteric priestly system, viz., that these 



family quarrels should not be produced before the 
general public, but should be fought out on the special 
ground, and in the serene solitude of scientific publi- 
cations. If a movement of this kind has already taken 
so wide a range, if the fight for truth has already 
reached such a pitch, it seems much more agreeable to 
the interests of both sides to lay the matter in dispute 
openly and clearly before the eyes of all men, and 
to give to those afar off, who may nevertheless hear 
of the contest in this direction or in that, a clear idea 
of the position of the contending parties and of the 
condition of the strife. 

If we compare closely our new theory of Evolution 
with the histories of creation given in earlier times, 
we find the fundamental principle of Darwin by no 
means a new one. It has been formulated already by 
many philosophers, not only in our own century, but 
in much earlier times, in one form or another. The 
proofs and arguments that Darwin has discovered in 
favor of his views are new. The vigorous carrying 
out of the hypothesis in the light of the science of 
to-day — that also is new. If we study all the earlier 
theories of creation, we can arrange them all into two 
distinct series — (1) Cosmogonies that hold, with the 
Mosaic account of creation, that all kinds of living 
beings have been called into existence independently, 
separately, at the will of an almighty creator ; (2) Cos- 
mogonies that regard all living things as branches of a 
single stem, products of one eternal law of Nature, the 
law of progressive Evolution. With these two funda 
mental ideas a whole series of antagonistic theories are 
bound up inseparably, and in most characteristic 
fashion. Each of the two orders of thought has utilised 
the extraordinary acquisitions made by science during 
the last century in entirely different manners. Each 
has used them for the building up of its own special 
system. Out of these many acquisitions my purpose 
will be to glance at those dealt with by geology, the 
study of the construction and of the origin of the 

According to the generally accepted idea, that globe 
was in very early times a fiery, liquid ball, whose 



surface, cooling down, hardened into the crust of the 
earth. When the temperature had sunk to a certain 
level the hot vapors condensed in the form of water. 
Then came the first possibility of the existence of living 
beings on the cooled, hardened crust of the globe. The 
beings that in those early, far-off ages peopled this 
earth through many, many millions of years took origin 
from organised matter of a much lower kind than that 
of which the majority now in existence consist. Of 
many important divisions in the vegetable and in the 
animal kingdom for a long period no representatives 
appeared : of other divisions only the simplest and 
most imperfect forms existed. In the course of the 
measureless ages that have passed since that time series 
on series of beings have evolved, approaching gradually, 
step by step, in ever-growing perfection and complexity, 
the fauna and flora of the world of to-day. Stratified 
rocks deposited in the water investing the originally 
naked crust of the cooled globe, teach us that the surface 
of that globe had to pass through many a change in the 
course of these long ages, and underwent many a rise 
and fall. As result of volcanic and meteorologic in- 
fluences the earth-crust split now here, now there, and 
ere long one or another region sank beneath the water, 
and anon rose again from the waves. Dust and stones, 
the fragments of rock, worn, broken, pulverised by 
wind and water, aggregated into the form of mud and 
sand at the bottom of the waters, enclosing within 
themselves the remains of dead organisms. 

These valuable fossil remains, as petrified animals 
and plants, or the imprints of these, afford disclosures 
of the vastest importance as to the history of the earth, 
the succession of organisms, and the nature of the living 
beings that have dwelt upon its surface. The succes- 
sion of organisms wherein living beings appear one 
after another, and the demarcation of the many strata 
placed one upon the other, in which the former are 
preserved, have been explained in very different w T ays. 
Following Cuvier and other naturalists of the foremost 
rank, men held generally, towards the end of the last 
century and until the fortieth year of the present one, 
that a succession of entirely distinct earth-periods had 



followed one another, each with its own special popu- 
lation of living things. The surface of the globe had 
from time to time been changed by vast cataclysms of 
unknown origin, in such fashion that on each occasion 
the world of living things had perished wholly, or to a 
great extent. After each revolution of this kind every 
group of animals and plants that came into view must 
therefore owe its origin to a special act of creation. 
With this idea, that the animal and plant world of a 
given creation-period has been created independently, 
and without any connexion with those that preceded 
it, is closely connected a second, very influential opinion, 
introduced by Linnaeus, and specially followed up by 
Cuvier. It is that all organic individuals that we en- 
counter in Nature can be arranged in groups that bear 
the title of species. What is a kind or a species ? No 
scientific man has hitherto been able to give a well- 
grounded and satisfactory definition of the word. 
According to the opinion of the majority all those 
individuals, as, e.g., all horses, all apple trees, belong 
to one kind or species, which either originate from a 
single pair of ancestors, or (as this generally does not 
admit of proof) agree in all essential particulars, and 
only differ in points of secondary importance. Every 
species, indeed, within defined limits may appear 
variable, and may vary. Thus, in the species Horse 
many different races, in the species Apple tree a 
series of varieties of apple can be distinguished. But 
all these races and varieties of one species ought never 
to be separated from one another by essential marks as 
are all the closely related species of one genus — e.g., 
horse and ass, or apple and pear tree. On the other hand, 
if we consider certain resemblances, some close, some 
more remote, and if we study the marks of agreement 
among living beings, we are able to group together 
several species into one genus, many genera into a 
family, and allied families into a class. But these 
divisions are generally regarded as arbitrary, whilst 
much strife reigns as to their limits and extent, and as 
to whether the idea of a species is a precise one and 
really founded on Nature. "There are as many species," 
says Linnaeus, " as the spirit of god in the beginning 



created living beings." Or as Agassiz expresses it : 
" Every species is an embodied thought of creation." 
On this conception, on the dogma of the constancy of 
species, is laid the foundation of the theological cos- 
mogonies. Every species is regarded as an independent 
unity, unconnected with all other species, and endowed 
by a creator with qualities and instincts adapted for 
its particular mode of life. 

The followers of the philosophical Evolution theory 
take a very different view. According to them, the 
different periods that the older thinkers regarded as 
sharply-defined sections of the earth's history are not 
separated one from the other by definite limits. They 
glide one into the other, after the manner of the periods 
that we recognise in human history. Even as now, 
each age bears its individual character. Two successive 
sections of time are never separated by a violent 
cataclysm, destructive of the existing living world and 
necessitating a new creation at the beginning of a new 
period. Further, a direct connexion is always evident 
amongst organic things. A larger or smaller proportion 
of the living beings passes over unchanged from every 
older period to the next youngest, although their strata 
seem so sharply defined. But even the new animals 
and plants that seem to appear on a sudden in the later 
strata are so closely allied to certain others in the pre- 
ceding stratum, are in certain respects so like these 
that the conclusion is well founded, that they have 
originated directly or indirectly from their predecessors, 
and only represent the sub-species or varieties of those 
earlier species modified and adapted to changed con- 
ditions of life. This idea attains its full development 
in the conception that is its necessary result, that all 
the organisms of any given earth-period have arisen 
from the organisms of preceding periods, all the living 
things of the world of to-day from those of the world 
of yesterday : that the organic life now existing is con- 
nected indissolubly by real, genealogical blood -ties 
with all animals and plants ever existent on this earth 
in earlier ages. The fact that the history of the evo- 
lution of earth shows us a continual, uninterrupted 
advance in its population, a continual increase of 



organised species in number, complexity, structure — 
this in addition to a series of other geological truths, 
the discussion of "which in this place would carry us 
too far — forces us to the belief that all these different 
species have evolved from a few, perhaps from one 
original primordial form by way of natural descent 
that has been accompanied by continual advance in 
structure. The whole of the natural system of plants 
and animals appears from this point of view as a vast 
genealogical chart, and may, like every family pedigree, 
be best imaged under the similitude of a wide-spread 
tree, whose simple root lies hidden in the remotest 
past. The myriad green leaves that cover the younger, 
more recent branches, and spring out from the central 
stem in all varieties of shape, represent the species of 
plants and animals now living, the more perfect the 
farther they are distant from the original stem. The 
dried, withered leaves, on the other hand, on the 
older, dead branches represent the many lost, dead 
species that dwelt in earlier times on the earth ; and 
the more they resemble the original simple primordial 
form the more remote are they. No species, not even ^ 
the first, has been independently created. They have 
all originated in the course of immeasurable ages from 
a few or from one simple spontaneously-arising original 
form' that has obeyed one law of Evolution — a law that 
works slowly and gradually, but ceaselessly — a law / 
that leads to higher and higher levels of perfection. 
Thus the idea of a species is as changeable, as arbitrary, 
as little absolutely conclusive, as the more general and 
larger conceptions of the genus, the family, the class. 
New species can only arise from existing species. 

At the beginning of this century this idea, that at the 
first blush seems so strange, had already dawned upon 
several eminent minds. Even whilst Cuvierwas framing 
his system, a foe had already appeared who threatened 
to pluck up his whole crop by the roots. This was the 
celebrated French scientist, Lamarck, who as early as 
1809 published, in his remarkable " Zoologie Philoso- 
phique," a theory thoroughly thought out, as to the 
origin of organic species by the gradual variation of a 
few spontaneous original forms. The school of think- 



ers whose most important adherents were Geoffroy St. 
Hilaire in France, and Oken in Germany, joined him. 
With prophetic insight these deep-thinking men moved 
in advance of their own time, and, in truth, held the 
very views on the actual blood-relationships of organ- 
isms whose scientific foundation upon facts was 
reserved for Darwin and his followers to build up 
during the last four years. At that time experimental 
facts were on the whole wanting to these men, espe- 
cially those relating to the embryonic and palaeontolo- 
gical development of organisms, that we at the present 
day know with comparative accuracy. It is no wonder, 
therefore, that they often went beyond the limits of 
experimental inquiry, and thus furnished their rigidly- 
accurate opponents, Cuvier and his disciples, with not 
a few weak points for attack. The contest between the 
two opposed orders of thought, carried on with much 
ability on both sides, reached its height in a violent 
discussion at an open sitting of the French Academy, 
on February 22nd, 1830, between Geoffroy St. Hilaire 
and Cuvier. It ended in a victory for the latter that 
lasted the next thirty years. This remarkable contest 
has been described and criticised in the most striking 
and ingenious way by Goethe, in one of his last essays, 
written only a few days before his death. Goethe 
followed the philosophical contests of his time with 
the keenest interest even in his later years, and in this 
dispute he sided with Geoffroy, and was opposed to 

For thirty years from that time Cuvier's theory, 
rigidly limited by experimental inquiry, held sway, 
until, in 1859, it was shaken to its foundations by the 
epoch-making work of Charles Darwin " On the Origin 
of Species in the Vegetable and Animal Kingdom by 
Natural Selection," or by the survival of the fittest 
races in the struggle for existence. That which is 
really new and valuable in the Darwinian theory is 
this : that the correct theory of Evolution of the earlier 
philosophers, mingled as it was with many errors, was 
cleared of these erroneous elements, and in addition 
was supported by arguments based on fact, in part 
altogether original and peculiar to their enunciator, in 



part borrowed from the antagonistic works of Cuvier 
himself. Geoffroy and the other philosophers derived 
the manifold likenesses and relationships with which 
the bodily structure of organisms presents us from one 
common archetype that underlies all organisation. The 
differences that accompany these resemblances would 
arise in the course of the reproduction of the species, for 
whilst certain organs would advance, others would 
retrograde. This derivation of the resemblances, or 
homologies, from the principle of a common origin, 
Darwin uses. But he avoids the limited application of 
the principle by connecting with it, in a manner most 
fruitful in results, an idea of Cuvier that is apparently 
opposed to the principle. The idea is that every 
animal and every plant, independently of any common 
archetypal plan, has received, or better, has acquired 
a special organisation adapted to its conditions of life ; 
not that its size, color, form and internal structure, 
have been adapted to the manner of life appointed for 
it by the creator, but rather that the living being has 
adapted itself to the conditions of existence of its 

The great importance of that agreement in ground- 
plan of organisms that without doubt does exist (the 
unity of organic composition, as Geoffroy calls it) is, 
very differently from Cuvier, acknowledged and valued 
by Darwin. But by him this idea is at the same time 
explained in simplest and most natural fashion. He 
traces it back to the fundamental, momentous principle 
of heredity. Heredity is a property common to all or- 
ganised bodies, a universal law of organic Nature, the 
lofty significance of which we generally overlook, 
because in our every-day life its working is at all times 
and in all places before our eyes, and in its action we 
ourselves are included. From our childhood we are 
so accustomed to it that we wonder at deviations from 
the principle of heredity more than at heredity itself. 
It is well known that every pair of human beings 
transmit to their offspring not only the general pro- 
perties of the human organism, but also a certain 
quantity of peculiarities of body and of mind that to 
some extent distinguish very definitely the various 



members of a given family from other human beings. 
For example, in some families six fingers are congeni- 
tally present on each hand. So generally is it recog- 
nised that the color of the hair and of the eyes, the 
outline of the face, and further the specially mental 
qualities of temperament, disposition, force of will, are 
transmitted within the limits of the family, that any 
further enumeration of examples in this connexion is 
unnecessary. On the other hand, it is equally well 
known that this same heredity is never absolute and 
unconditioned. It is only relative. However the 
children of the same pair of human beings resemble 
each other, however closely their bodily and mental 
natures agree, they are nevertheless distinguished from 
each other and from their parents by certain special 
peculiarities only belonging to the individual, that we 
therefore call " individual qualities." These specialties 
are in some degree established even in the egg or germ 
of the individual. So that all organisms are compelled 
by the law of heredity, and also by a general law of 
variation, to transmit to their offspring their peculiar 
character of disposition or of faculty, unchanged to 
some extent, but altered in greater or less degree in 
other particulars. To some extent, also, individual 
qualities are acquired for the first time during the life 
of individuals, through adaptation to the conditions 
of life, and especially through the changing relations 
in which every organism stands to all others that 
environ it. 

Whilst those qualities, transmitted and inherited from 
ancestors, heirlooms in a particular family through 
many generations, are reproduced in the progeny, it 
frequently happens that special variation, appearing 
suddenly for the first time in one individual or sud- 
denly acquired by it, is transmitted in like manner to 
the offspring of that individual, and thus becomes the 
possession of a series of beings, of a complete division 
of a family. This happens frequently with certain 
consumptive diseases and with peculiar mental dis- 
turbances. In the first instance the abnormality 
appears as a deviation from the law of heredity. 
Next it is itself subjected to this very law. If we 



wish hereafter to trace out carefully the genealogy of 
any great human family of which we have, in addition to 
the name, a brief account of each member, we shall find 
that the original family marks disappear more and more 
as we descend from the primal ancestors to the later 
descendants. The larger the number of these, the more 
numerous the generations intervening between them 
and the ancestral forms, the more are they differen- 
tiated, the more widely apart are they in many respects, 
the more completely are the old hereditary family 
marks eliminated and replaced by new peculiarities, 
partly inherited from younger ancestors, partly acquired \S 
anew. We shall be able to separate on the genealogical 
tree certain groups and sub-groups that we can connect 
by radiating and divergent lines of relationship. 

It is evident that the same kinship that among men 
connects the individuals of one family and of one name 
is widely spread in the vegetable and in the animal 
kingdom. In these each individual has certain cha- 
racters inherited from its ancestors, certain others that 
it has independently acquired, and that it can transmit 
to its offspring. The principle of heredity in these 
cases is still dominant, and has been used by many 
scientists to maintain the idea of a kind or species. 
According to a view widely held, those individuals, 
and only those that have originally sprung from one 
pair of ancestors, should constitute a species. This 
definition is not only accepted, but is widely extended, 
by Darwin. He not only believes, in fact, that all the 
members of a species have had a common ancestor, but 
he asserts the same truth of all the species of a genus, 
and, yet further, of all the genera of an order. Finally, 
all the orders of a class — e.g., the class Aves — must 
have taken origin from one common ancestor lying 
far back in the series of periods of creation, and the 
separate ancestors of all the classes must have sprung 
from one most simple form. On this same principle of 
heredity new species can continually arise. It is well 
known that in many species certain groups of indi- 
viduals are distinct, often very markedly distinct, from 
one another. Hence it follows that these groups are 
distinguished as races, sub-species, varieties. But the 



difference between the varieties of a species ought never 
to be so great as that between two nearly allied species. 
Darwin contradicts this idea ; for if once a very strongly 
aberrant variety, or a single monstrous individual, has 
presented a certain variation so marked that the dif- 
ference between it and the parent species is greater 
than the difference between the latter and the species 
most nearly allied to it : if, further, the strongly aber- 
rant individual in turn transmits this special variation 
to its offspring : if, finally, this variation maintains 
itself unaltered through several generations and be- 
comes permanent — in this way a new independent true 
species is constructed from the variety or race of the 
original older species. Thus, by heredity, many species 
may originate from a single species. 

In what circumstances is a new species thus suddenly 
appearing actually maintained, and under what con- 
ditions does it find a footing independently of its parent 
form ? Darwin's special work has been to place this very 
momentous question in a light wholly new and clear. 
When we study these conditions we reach the central 
point of the Darwinian teaching in the investigation of 
the immensely important successive changes of organ- 
isms that he names " Struggle for life " and " Natural 

Darwin starts here from the weighty truth that all - 
organisms propagate themselves by offspring, whose 
number constantly increases in more or less swift 
progression. All animals and plants, without exception, 
strive to multiply to such an extent that, left to them- 
selves and protected from all adverse influences, each 
would completely possess and people the region best 
fitted for its existence in a very short space of time. 
The whole surface of the earth would be occupied, e.g., 
after a few years, by the progeny of a single pair of 
mice, who multiply very rapidly. Even of the elephant, 
which reproduces itself most slowly of all animals, the 
offspring of a single pair would in five hundred years 
amount to the enormous number of fifteen millions. In 
this calculation the minimum is taken — viz., that each 
pair of elephants, during their whole life of ninety 
years, only bring six young into the world. Among the 



lower animals, on the other hand, are many even among 
the class Pisces, of -whom each individual produces not 
merely a hundred or a thousand, but even a hundred 
thousand or a million eggs. In all cases only a very 
small proportion of these germs come to maturity, so 
as to serve for the propagation and the maintenance of 
their race. By far the largest part perish early. The 
reason of this fact is very simple. Only a limited num- 
ber of places exist in the vast household of Nature. 
Only a limited number of organisms can exist at the 
same time on the restricted space of our globe. In a 
field of a certain size only a certain number of seeds of 
one or of several kinds of plants can be sown, and only 
a part even of these ripen. Of the buds only a small 
number will flow T er, a still smaller number fruit. The 
majority of the seeds sown are carried off by birds and 
other animals. A thousand foes lie in wait for the 
young bud that has struggled upwards out of the earth, 
the more numerous and the more formidable the 
younger the delicate bud may be. Many of the 
young plants therefore perish during their growth, as 
they are outgrown, supplanted, arrested in their de- 
velopment by others of their own kind. Between all 
allied individuals a constant strife, a battle goes on 
for space for their roots, for moisture, light and heat — 
a battle in which the weaker ones must perish. 

The w T orld of Nature, as a whole, shows us the same 
conditions. More than a certain number of living 
beings can never reach their full development, whilst 
by far the greater part perish early. In most cases 
the struggle for existence is very complicated, and 
generally a number of different species of animals and 
plants living in the same place are connected in con- 
flicting ways that are as a rule only very imperfectly 
known to us. Thus carnivorous animals have great 
influence upon the existence of certain plants, for the 
insectivorous animals, that serve for the most part as 
food to the carnivorous ones, affect especially certain 
carnivorous beetles. But the latter live chiefly on 
certain other insects, that in their turn are dependent 
upon the particular plants under consideration. In 
this chain each link is dependent upon the next. As a 



good example of this relationship, that is often very 
complex, Darwin quotes the effect that in England cats 
exert on the formation of seed in red clover. The buds 
of the red clover belong to those flowers that can only 
be fertilised by aid of certain insects. In this special 
case the necessary insects are the humble-bees. The 
chief foes of the humble-bees are the field-mice. Now 
where cats most do congregate, who kill a large number 
of mice, the humble-bees will be more numerous, and, 
therefore, the clover will more frequently attain fructi- 
fication. In a similar web of manifold relationships is 
every animal and every plant linked on to all others 
that live in the same place. In most cases these rela- 
tionships are wholly unknown to us, but that they exist 
everywhere may be asserted with certainty. As long 
as each individual requires a certain amount of food, 
a certain position or standing-ground, it must neces- 
sarily come into collision with many neighbors striving 
after the same end. In Nature, as in human society, 
reigns everywhere a battle of all against all, remorse- 
less and unceasing. And as the number of places in 
the world is limited, as space and food only exist in 
sufficient quantity for a very small proportion of the 
germs, the majority must of necessity perish. 

Now it is clear that, on the average, in this fight for 
existence, those individuals of the same species will 
conquer and outlive others that are in any way better 
organised, possess more strength to withstand their 
adversaries, greater readiness to beget offspring, or in 
some other way have, through any special quality of 
organisation, an advantage over others. On the whole, 
it will always be the weaker and worse individuals that 
succumb and die out, the stronger and better that sur- 
vive and propagate their kind. As this advance is 
repeated by the same species through many generations, 
a continual advance in perfection of organisation must 
result. Truly, in each individual case the approach 
towards perfection is but slight, and generally not 
noticeable. But if this slight improvement is repeated 
very often, at last, as result of continued increase and 
of accumulation, the many small improvements produce 
so much more advanced a condition of the organism 



that the final term of the long series differs very widely 
from the first. 

This advancing improvement of the species confirms 
in remarkable fashion the opinion that fits in with the 
tendency to variation explained above, and with the 
general habit of all organisms not to transmit the sum- 
total of their qualities to their offspring, but to alter 
them within certain defined limits. It has been already 
shown that these modifications extend so widely in 
many species that we can mark off in each species a 
number of different races or sub-species. Now it is 
clear that these, like the single dissimilar individuals, 
will be differently situated in the fight for life. One 
race or variety will be more, another less gifted, as re- 
sult of its special qualities, and it must follow that, if 
several races of the same species struggle for existence 
one with another in the same place, the races that are 
stronger, better developed, or, as consequence of certain 
properties, better fitted for existence, will overcome and 
supplant the other races that are weaker and less 
favored. In this way, according to Darwin, arises, e.g., 
the sympathetic coloring so frequently to be seen in ani- 
mals, i.e., the relationship between the color of their 
skin and that of their habitat. Beetles, aphides, and 
other insects living on leaves appear green ; bark-eating 
insects grey or brown ; butterflies and other insects 
that live on parti-colored flowers are colored in varie- 
gated fashion. The inhabitants of the vast steppes and 
deserts, e.g., the gazelles, jerboas, jackals, etc., imitate 
very closely the yellow or brown-yellow color of the 
sand. The majority of the polar animals are white, 
like the snow and ice on which they live. Amongst 
these animals are many, like the snow-hares, the ptar- 
migan, the polar fox, that are white in the winter, when 
the whole land is covered with snow, but in summer, 
when the snow is partially melted, are colored grey or 
brown. These remarkable sympathetic colors are easily 
explicable, on the ground that they are very useful to 
the animals concerned, and give them a distinct ad- 
vantage over individuals that are colored in different 
manner. Evidently, those individuals whose color 
differs least from that of their habitat will be least 




easily seen by others that hunt them or use them for 
food. They can more easily escape their pursuers, 
more easily approach their prey, than other individuals 
of the same species that, on account of their color con- 
trasting with that of their surroundings, will be more 
noticeable and more easily seen. Further, if at first 
many varieties of a given species are living hard by 
each other with different color-markings, later those 
that have an advantage through their particular hues 
will inevitably take the place of and conquer the others. 
In like manner many peculiarities apparently accidental 
are understandable on the view that they give to the 
beings or to the varieties that possess them an advan- 
tage over others of the same kind. These last, not 
possessing these advantages in the struggle for exist- 
ence, must go down before their more favored rivals. 

Darwin calls this immensely important principle 
Natural Selection. He compares it to the artificial 
selection that man is constantly exercising in relation 
to domestic animals and plants. If we look closely 
at this latter selection we find that the aim of the 
breeder is not simply confined to producing and pro- 
pagating races that are good, useful, profitable to him. 
It embraces the creation and the bringing up of yet better 
and more useful varieties than had existed. It in- 
cludes in fact, to put it shortly, the improvement of 
the race. This aim the breeder attains to some extent 
by using for breeding and propagation only the best 
and fittest individuals, or if he is following up some 
special plan, only those that show the peculiarity he 
desires in special degree. Thus the gardener only takes 
for planting the seeds of the best and strongest plants. 
The farmer selects carefully for breeding only those 
animals distinguished from others of the same herd by 
size, speed, strength, or any other special quality that 
he desires to bring out. These individual advantages 
reappear as a rule in the offspring, and generally in 
unequal degree in the different offspring. If, again, of 
these latter those are selected for the propagation of the 
next generation that exhibit the required speciality 
most clearly marked, and if this method is repeated, 
the advantage becomes yet more strongly shown in the 



descendants ; and if the same careful selection is carried 
on through many generations, the later progeny are 
finally improved to so great an extent that the form 
most recently evolved does not seem to belong to the 
same species as the far more imperfect ancestral forms. 
The differences between the various races have grown 
so marked that we should certainly regard them as quite 
different species, or even as different genera, did we 
not know that they have arisen from one and the same 
parent species by continuous differentiation, and are 
joined on to that parent species by intermediate links. 
The majority of our domestic animals have already in 
this manner departed so widely from their wild origi- 
nals, that we are altogether in the dark as to the latter. 

It is clear, according to Darwin, that this same suc- 
cession of changes, that man in these cases arbitrarily 
causes and directs, occurs continually among animals 
and plants that exist in a natural state, to the profit and 
steady improvement of these creatures. New races of 
a more perfect nature constantly arise and improve 
themselves in the struggle for existence, whilst the less 
perfect races (like the old parent forms) retrograde, 
become extinct, and die out. The selection of the best 
and fittest individuals for breeding, that results from 
the artificial selection in accordance with man's will 
and idea, is attained in Nature in natural breeding as 
result of the necessity of changed relationships between 
all organisms, as result of the conditions laid on each 
by the life-combat. But variations in the species do 
not, as in artificial selection, ultimate in the advantage 
of man, but in that of the various animals and plants 
themselves. The struggle for existence is so general, 
the effect of all organisms one on the other so compli- 
cated, the number of antagonistic individuals so great, 
that only those individuals that are specially favored 
can survive the fight, in which by far the larger num- 
ber of the weaker and unfit perish. It is incontestable, 
if this advancement is considered as a whole, that from 
it must of necessity result a constant, gradual alteration 
of the world of living things, a progressive metamor- 
phosis, an advancing transformation and improvement 
of all organisms. The lower, imperfect forms will con- 




tinually disappear : the higher, more perfect ones will 
endure. These will give origin to an increasing number 
of yet more advanced forms by means of lasting varia- 
tions, and divisions into new varieties will result. 

Every zoologist and botanist believes that in this way 
varieties and races are unceasingly arising. The capa- 
city for variation of the species is unlimited. The 
majority of thinkers only oppose the further extension 
of this succession of changes asserted by Darwin. He 
holds that new species and genera arise in exactly the 
same way. Yet further, he concludes by analogy that 
in kindred fashion also genera have evolved from 
orders, and orders from classes. Frankly, we are not 
in a position to confirm these latter conclusions by 
actual observation. For, although Natural Selection is 
at work continually and everywhere, seizing upon every 
fitting opportunity that the variation of species and the 
struggle for existence afford for the origination of new, 
independent forms, yet, on the other hand, it works 
too slowly, too gradually, and requires generally too 
long a time for the result of these constant transforma- 
tions to be visible to us. Natural Selection also seems 
to require a much longer time for the production of a 
form so marked that it can be regarded as a good species 
than artificial selection, in which many circumstances 
combine to favor the strengthening of the new form. 
But if many generations are necessary for the evolution 
of a new species as result of gradual modification, it is 
beyond doubt that the time needed for the production 
of a complete genus, order, or class, from one primor- 
dial form is wholly beyond our power of comprehen- 
sion. For such a series of evolutions epochs, not of 
hundreds and thousands, but of hundreds of thousands 
and millions of years are necessary. But the earth's 
history as a whole, from the appearance of the earliest 
original forms of simplest organisation to the rich and 
manifold series of the organic world of to-day, our 
knowledge shows to be made up of a series of such 
epochs of astounding length. Compared with the 
eternity of these vast periods of time, that we may 
estimate approximately whilst we can never really con- 
ceive them mentally, their latest moment, the many 



thousands of years since man, last link of the chain of 
being, appeared, fades into nothingness. The irre- 
fragable evidence of geology gives us proof of these 
enormous lapses of time. 

Although at the present time complete and actual 
proof of the origin of the larger groups of species from 
a single species is not furnished as result of direct ob- 
servation, yet we are acquainted with a vast series of 
facts that bear convincing evidence as to the truth of the 
Darwinian theory. Numbers of the most important 
natural phsenomena, inexplicable without its aid, find 
in it a simple, harmonious explanation. First amongst 
these ranks that gradual advancing development 
through which the organic world passes in the succes- 
sive organisms of successive periods of time. In the 
oldest strata in which clearly recognisable fossils are 
generally found, only very few and very simply orga- 
nised representatives of the great sub-divisions of the 
vegetable and animal kingdom have been discovered. 
He that passes upwards step by step in the series of 
strata sees that these low, imperfect beings give place 
to many higher and more perfect ones. Not only is 
there increase in the number of organisms in the more 
recent epochs near the present time, the simple forms 
are replaced by others more complex, more differen- 
tiated. Thus, e.g. of the Yertebrata, we find in the 
oldest fossiliferous rocks only rudimentary cartilagi- 
nous fishes. A little later their place is taken by 
higher fishes that approach more and more nearly to 
the majority of the osseous fishes now living. These 
are followed by the Amphibia (Labyrinthodonta), later 
by the Reptilia, especially the gigantic lizards. After 
these cold-blooded quadrupeds a very slow and very 
long Evolution leads up to the bird-like forms or flying- 
lizards, to the Deinosaurians, heavy brutes resembling 
the Pachydermata. Finally, the higher Yertebrata, the 
warm-blooded birds and mammals appear in the 
younger formations. At first, even of this last class 
only kangaroo-like marsupials occur. These hold the 
lowest place in the Evolution of this class, and from 
them are developed very gradually the higher, more 
perfect Mammalia that reach at length their loftiest 



height in the Evolution of the anthropoid apes, and at 
last of man himself. 

Considering all that we know of human existence on 
the earth, we are entitled to hold that man also did not 
spring, Minerva-like, full-armed from the head of 
Jupiter, nor come from the hand of a creator as an 
adult and sinless. He has worked upwards very 
slowly and gradually from his primitive state of un- 
civilised brutality to the first dawn of culture. To 
this, the more recent discoveries in the field of com- 
parative philology, in addition to the various facts 
brought to light by the geological and antiquarian 
research of later times, bear strong testimony. Speech 
did not appear suddenly, immediately, all at once, as 
the complex function of which man generally boasts 
as the special advantage possessed by him as compared 
with lower animals. More probably speech arose 
gradually from a few, simple, crude, animal sounds 
that served to designate the nearest objects, the most 
pressing wants. In a form little more perfect than 
this, speech still remains amongst a few races of 
lowest rank. The number of these expressions grew 
very slowly. At first they were by degrees combined 
into words, later into simple sentences. But a long 
period must have elapsed ere from this one, or from 
these few simple rudiments by advancing development 
and differentiation, the many stocks and families of 
languages evolved. For these are arranged by com- 
parative philologists in a system, divided and sub- 
divided according to their relationships near or remote, 
in the same way as botanists and zoologists arrange 
the families of plants and animals. As with the re- 
lationships of these last, those of speech can only be 
explained and understood on the principle of a common 
origin and progressive Evolution. We find the same 
law of advance at work still more widely in the historic 
development of the human race. Yery naturally. For 
in the relationships of the state and of society occur 
again the same principles of the struggle for life and 
Natural Selection that irresistibly urge forward and 
raise by degrees to a higher culture the nations of the 
earth. Retrogressions in the life of states and societies, 



of morals and of knowledge such as the combined and 
selfish efforts of priests and despots in all ages of the . 
world's history have produced, seem at times to hinder 
or apparently stop this universal advance. But the 
more unnatural, the more of an anachronism these re- 
trograde attempts are, the more swiftly, and with the 
greater energy follows the reaction, the advance that 
treads ever on their heels. For progression is a law 
of nature that no force of man, neither the arms of 
tyrants, nor the curses of priests, can ever long restrain. 
Life and Evolution are only possible with advancing 
movements. Already to stand still is to step back, 
and every step backwards has within itself the be- 
ginning of death. The future belongs to progress *y 

While the fact of an advancing Evolution is easily 
explained on the theory of Darwin, the latter agrees 
further with the no less important fact that all things 
living to-day, or that ever have lived, form one vast 
whole, a single wide-branching tree of life of great 
antiquity, whose various parts are not isolated or 
separated by sharp gaps even in the finest branches, 
but are directly connected everywhere by intermediate 
links and by transitional forms. In this connexion 
the study of fossil animals and plants is a necessary 
complement to the natural history of the living world 
of to-day. For many living beings that in their ex- 
ternal form and internal organisation seem to be 
widely asunder to-day, are closely connected by a 
chain of intermediate connecting links that lived in 
some cases very far back in the history of the earth. 
If we wish, therefore, to construct a so-called natural 
system of living beings, the extinct fossils must be 
considered as well as forms that are now living. If 
this be done, the whole natural system appears as one 
large organised body of many members, as a wide- 
branching tree, all the branches of which, its divisions 
and sub-divisions, are connected by radiating lines of 
union gliding one into the other. This fact, at first so 
surprising, can be explained by no other hypothesis 
save by Darwin's conception of a common origin for 
all. The mighty tree, wide-branching, under the 



similitude of which the natural system is best repre- 
sented, attains its fullest significance as one huge, 
universal genealogical tree of all animals and plants. 
The word " Relationship " no longer remains, as 
hitherto, a merely figurative expression to indicate the 
degree of resemblance or dissimilarity between living 
beings, but acquires its full primitive, real significance 
when it reveals to us the common origin of these 
beings from one ancestral form, i.e., their veritable 
blood relationship. Already the more or less close 
relationships that exist between groups closely allied 
and subordinate one to the other, have been denoted 
by the phrase "natural affinity," without anyone anti- 
cipating that this imaginary arrangement expressed the 
truth as to those relations in the only right manner. 

Whilst the hypothesis of Darwin yields us the key 
to the enigma of affinity, it explains the majority of 
the rest of the phenomena of organic nature in a 
fashion as simple as it is effective. As examples, I 
quote the remarkable facts as to the geographical dis- 
tribution of animals and plants, the phenomena of the 
division of labor, alternation of generations, metamor- 
phosis, the meaning of rudimentary organs, which are 
as important morphologically as they are worthless 
physiologically ; lastly, and most significant of all, the 
very momentous threefold parallel between the em- 
bryological, systematic and palseontological Evolution 
of organisms. It is not possible, on account of the 
limited time at our disposal, to go more fully into these 
three parallels that I, for one, regard as the strongest 
proof of the truth of the Evolution theory. These and 
many other deeply interesting phenomena that were 
described by the earlier naturalists as " curious freaks 
of Nature," without the aid of the theory of evolution 
seem as strange, insoluble riddles to us. By its means 
they are all explained from one and the same point of 

On the other hand, we ought not to forget that Dar- 
win's theory of Evolution does not by any means give 
us a system matured and finished. It offers us only 
the basis of a future system. It gives the first great 
impulse towards a thorough reform of the present 



system. Many gaps and weak places in the growing 
edifice give large opportunity to the attack of foes. 
On the other hand, many circumstances are still alto- 
gether, or almost altogether, unknown to us, that are 
possibly of no less moment in the origin of species 
than Natural Selection in the struggle for existence, 
on which overstress is, perhaps, laid by Darwin. No 
less important than these must be those external 
conditions of existence of the inorganic world that 
are too much neglected by Darwin, climate and 
habitation, and those geographical and topographical 
relationships, to which the characters of organisms in 
many ways become adapted. 

Another want, and perhaps the most important in 
the Darwinian theory, lies in the fact that it yields us 
no starting-point for the spontaneous appearance, or 
origin of the one or few oldest forms from which all 
the rest have evolved. Was it a single cell, like those 
that even now exist as independent beings on the 
dubious borderland of the plant and animal kingdoms, 
or such as the ova of all organisms present at some 
period of their existence ? Or was it at a still more 
early period, simply a mass of living protoplasm, 
capable of digestion, reproduction, Evolution — one of 
the Monera, allied to certain amoebiform organisms 
that do not seem as yet even to have risen as far in 
organisation as the cell ? 

To these and other questions the new aspect of the 
Evolution theory that is due to Darwin gives no 
answer. But this is not to be wondered at, if we 
remember that these inquiries have only been turned 
into this fruitful path by the epoch-making labors 
of Darwin during the last four years, whilst most of 
the earlier naturalists pursued aims of a totally dif- 
ferent character. Hence Darwin's new theory of crea- 
tion has met with many opponents of note among the 
older naturalists. But if we look back to the greatest 
discovery made by man — to the discovery of the law 
of gravitation among the heavenly bodies — if we re- 
flect that the discovery of Newton, universally accepted 
to-day, was in his day condemned and vilified as an 
error, harmful, revolutionary, heretical, not only by 



multitudinous priests and laymen, but even by philo- 
sophers and naturalists of note, like von Leibnitz, we 
shall assuredly cease to wonder that the same impotent 
anathemas assail the Evolution theory of Darwin, that 
theory which is the most valuable advance made by 
science in our time, that theory which gives promise 
of accomplishing for the organic world results akin to 
those effected for the inorganic by Newton's law of 

I therefore, as firmly convinced of the truth of the 
theory of descent as Darwin himself, bring to a close 
this imperfect attempt at offering an epitome of that 
theory by quoting the words with which Bronn, the 
translator of Darwin — although he acquiesces in the 
theory only conditionally — recommends the work of 
Darwin : " The probability of connecting, by aid of 
the Darwinian hypothesis, all the phsenomena of the 
organised world by a single principle, of regarding it 
from one point of view, of deriving it from one cause : 
the probability of intimately connecting many facts 
that have hitherto been isolated with others, of proving 
them to be necessary complements of those others : the 
probability of explaining most problems by it in most 
remarkable fashion : these stamp it as a pure truth, and 
gives us the right to expect that the difficulties that 
still present themselves against this theory will finally 
be overcome." 



AMONGST the notable efforts of the human mind 
that the long history of the Evolution of human 
knowledge places in strong relief, few have been of 
wider significance, of deeper influence than the 
Copernican system of the universe. For nearly 1500 
years the spherical astronomy of Ptolemy of Alex- 
andria had held sway over civilised humanity. In 
entire accord with things as they appeared to the 
senses, our mother earth was according to the Ptolemaic 
system the fixed, immovable centre of the whole uni- 
verse, and around it sun, moon, and stars moved in 
concentric circles. The movement of these heavenly 
bodies was from east to west, just as it may be seen of 
all men day by day. This account of the universe 
took root in the Christian world the more firmly in 
that it agreed completely with the words of the Bible. 
"In the beginning God created the heavens and the 
earth " are the first words of the first book of Moses. 
And the 16th verse of the first chapter says : And God 
made two great lights ; the greater light to rule the 
day, and the lesser light to rule the night : he made 
the stars also. And God set them in the firmament of 
the heaven to give light upon the earth." 

What could, in truth, stand more firmly, more se- 
curely than the Ptolemaic system ? " Does not the 
heaven arch over us up there ? Is not the earth lying 
beneath our feet ? Are not the eternal stars smiling 
upon us from on high ? " Could not every man of 
sense see with eye and touch with hand the earth 
standing still where she was, behold the sun, the 
moon, the stars palpably moving round this centre of 
the universe ? And this conception agreed with the 



place of man in Nature. Man, the veritable image of 
god, the be-all and the end-all of creation, was as 
certainly the one lord and head of the universe. 

After the long dark night of the mournful Middle 
Ages comes the dawn of the 16th century with its 
mighty advances, its heaven-scaling revolutions in all 
phases of the knowledge and of the belief of man. In 
this dawn the German Copernicus rises, a star of the first 
magnitude. His work on " The Revolutions of the 
Heavenly Bodies " led to the most decisive revolution 
in the thought of the world of his day. It is true that 
Copernicus did not himself behold the full result of 
his magnificent labors. The first printed copy of his 
work was seen of him only in the hour of his death. 
But many an earnest scholar and follower helped in 
the spreading of his views far and wide, and ere long 
a Kepler, a Galileo won for the Copernican system a 
complete victory. It is true that Tycho Brahe, dis- 
tinguished investigator and clear thinker as he was, 
tried to save the Ptolemaic system, or at least by com- 
bining it with that of Copernicus to find a happy mean 
between the two. The simplicity and clearness of the 
ideas of Copernicus, Kepler, and Galileo were so plain, 
their rigid, mathematical proofs so convincing, that ere 
long the momentous truth was clear to every inquirer 
who thought and was free from bias, that the earth 
moves. It turns each day upon its axis from west to 
east. It is a star among the stars, a planet in the midst 
of other planets that with the earth move round their 
common centre, the sun ; and round the earth wanders 
a single satellite, the moon. 

We can hardly picture the effect of these immense 
strides in the knowledge of Nature on the men of the 
16th and 17th centuries, who then were awaking for 
the first time from the long sleep of the Middle Ages. 
Not only did the rude, uncultured masses find a huge 
difficulty in this new teaching that turned the world 
upside-down, and contradicted absolutely the very 
evidence of the senses. Able, thinking men were un- 
able to dissever themselves from the old traditions so 
deeply rooted in their natures. And many of those 
who were clearest of perception, w T ho were compelled 


to admit the truth of the Copernican system, feared 
that the worst consequences would result from the 
spreading of the truth, and therefore sought to confine 
it within the narrowest possible limits. Especially did 
they dread the universal shattering of the Church 
theories that then lorded it over men, that was inevit- 
ably bound up with this new truth : for many neces- 
sary points of faith were overthrown by it, and the 
Bible would lose on many important points its abso- 
lute authority. Above all, the ambitious priests offered 
the Copernican system the most vigorous resistance, 
and tried to destroy their dangerous foe by the dicta- 
torial utterance of dogmatic points of faith. The 
whole arrangement of the universe, as perceptible by 
the senses, and with it all human morality, must perish 
if the Ptolemaic system went. The pernicious heretics 
who disseminated such immoral teaching must be ex- 
tirpated by fire and sword, and the ability shown by 
the Christian Inquisition in the invention of the most 
horrible tortures to the glory of god is too well known. 
Old Galileo, the greatest genius of his time, languished 
for a year in the dungeons of the Romishi Inquisition. 
He was forced to repeat once a week the seven peni- 
tential psalms of David, and kneeling before ignorant 
monks, his hand on the gospels, to forswear the eternal 
truth he had made known to man. But his proud 
speech " E pur si muove " (and yet it moves), spoken 
at once upon the formula of abjuration as he rose to 
his feet, has been from that hour the motto of all 
seekers after truth, who with a noble courage that at 
times seems almost too daring, have fought a way for 
the truths of Nature against priests and creeds. 

Every attempt to bid the world stand still has been 
in vain. "And yet it moves." Nevertheless a pro- 
tracted and obstinate resistance was made to the 
teaching of Copernicus, of Kepler, and of Galileo, and 
that from very influential quarters. But it arose in its 
strength, redoubled its vigor when Newton, the great 
Englishman, made that greatest of all human dis- 
coveries, that of the law of gravitation, and demon- 
strated in the force of gravity, the attraction of masses, 
the mechanical cause, simple as momentous of those 



movements of the planets that were recognised in the 
Copernican teaching. By this law the new mechanical 
conception of the universe was so strongly and un- 
answerably confirmed, an immutable law of Nature so 
clearly and simply demonstrated as the actual cause of 
the movement of the planets in their orbits, that it was 
necessary for the power of the priesthood anew to 
summon all its forces and set at work all its pens to 
oppose this heresy, so terrific, so scornful of all revela- 
tion. Here also, besides the ignorant and fanatical 
monks there were men of high culture and deep 
thought who aimed at the suppression of the free ex- 
pression of scientific knowledge. The most notable 
instance is that of the famous philosopher, Leibnitz, 
who condemned Newton's law of gravitation because it 
undermined natural, and denied revealed religion. 

We are reminded very vividly of these antagonisms 
and contests at the present day by the theory of 
Darwin, and the extraordinary stir that it has occa- 
sioned. It is in the first place clear that the main 
point of this theory, the question of the origin of 
species among plants and animals, has commanded an 
interest as widespread as the rotation of the earth and 
the movements of the planets. Every exhaustive and 
comprehensive investigation of that question soon 
demonstrates that it has an importance at least as great 
as these, and that the theory of selection of the 
Englishman Darwin is worthy to be placed side by 
side with the gravitation theory of his great country- 
man, Newton. And this is evident if we reflect upon 
the different meaning that Darwin's teaching has 
given to the so-called "history of creation" as a whole, 
and especially to the history of the creation of man. 

Darwin at first only attempted, in his celebrated 
work the " Origin of Species," to answer the question : 
" How have the different forms of plants and animals 
that we generally distinguish as kinds or species origi- 
nated ? " But this question is most intimately con- 
nected with two others of the highest significance, 
which in like manner must be solved along with it, 
viz., first, the general question : " How did life, the 
living world of organisms, arise ? " and, secondly, the 


special question : " How did the human race origi- 
nate ? " 1 

" The first of these two inquiries, that as to the first 
appearance of living beings, can only be decided empi- 
rically by proof of the so-called Archebiosis, or equi- 
vocal generation, or the spontaneous production of 
organisms of the simplest conceivable kind. Such are 
the Monera (Protogenes, Protamoeba, Protomyxa, Vam- 
pyrella), exceedingly simple microscopic masses of 
protoplasm without structure or organisation which 
take in nutriment and reproduce themselves by divi- 
sion. Such a Moneron as that primordial organism 
discovered by the renowned English zoologist Huxley 
and named Bathybius Haeckelii, appears as a con- 
tinuous thick protoplasmic covering at the greatest 
depths of the ocean, between 3,000 and 30,000 feet. It 
is true that the first appearance of such Monera has 
not up to the present moment been actually observed ; 
but there is nothing intrinsically improbable in such 
an Evolution. On general grounds it must be regarded 
as an essential to the beginning of living beings on the 
earth, as the point of commencement of both the veget- 
able and animal kingdoms. This idea is but a necessary 
sequence, the logical outcome of close reasoning. The 
second of the two questions that are necessarily con- 
nected with Darwin's teaching, that as to the origin of 
the human race, will alone concern us in this lecture. 

The solution of these two problems has hitherto 
been deemed so difficult by most naturalists that they 
have not ventured to attack it at all, or they have had 
recourse to some fundamental force of Nature of a 
special kind and wholly hidden from us. Not a few 
declared the solution of these queries to be quite impos- 
sible, and held that the origin of living things was not 
dependent on natural causes and could not be ascer- 
tained by science. Many more held that it could only 
be explained by the conception of a creative force 
above and external to Nature, that governs and has 
under its control the ordinary natural forces of matter, 
the physical and chemical forces. Some looked upon 
this unknown, enigmatical, ultimate supernatural 
creative force as the possession of a personal, more or 



less anthropoid, Creator. Others called it vital force, 
the organic principle of design, final cause, and so 

It is only necessary to hint at the fact that the 
accounts of creation of the religious teachers among 
different nations are always in conformity with the 
supernatural, mystical explanation just given. How- 
ever these may differ in particulars they all agree in 
this that they regard the first appearance of life on the 
earth, the origin of plant and animal species, and most 
of all the origin of the human race as a supernatural 
phenomenon that could not result from simple mecha- 
nical causes, from physical and chemical forces, but 
requires the direct intervention of a creative personality, 
working and constructing on a definite and designed 

Now the central point of Darwin's teaching — whether 
it has been already declared by the illustrious natu- 
ralist himself or not— lies in this, that it demonstrates 
the simplest mechanical causes, purely physico-che- 
mical phenomena of nature, as wholly sufficient to 
explain the highest and most difficult problems. 
Darwin puts in the place of a conscious creative force, 
building and arranging the organic bodies of animals 
and plants on a designed plan, a series of natural forces 
working blindly (as we say) without aim, without 
design. In place of an arbitrary act of creation, we 
have a necessary law of Evolution. By this the wide- 
spread incarnation of the divine creative power, or 
anthropomorphism, is done away with, the false idea 
that the creative force shows any likeness to human 
method of action. 

Clearly if these deductions are true the epoch-making 
work of Darwin must give the greatest offence and 
excite the most violent opposition on the part of all 
who are of opinion that without that unscientific idea 
of a supernatural act of creation the whole so-called 
order of the universe, as perceived by our senses, 
comes to an end. In the first place there is a revolt of 
all the scientific men who postulate an absolute dis- 
tinction between the inanimate and animate, between 
inorganic and organic Nature, and who consider that 


for the phenomena of the inanimate or inorganic world 
(e.g., for the movements of the planets and for the 
formation of this globe) exclusively mechanical causes 
or blind unconscious natural forces (causce efficientes) 
suffice ; but for the phenomena of the animate or or- 
ganic world on the other hand (the plant and animal 
kingdoms), causes that have a definite aim in view or 
conscious creative forces are required. In the second 
place these naturalists have as allies those priests who 
thought that the very essence of their power over the 
people was in danger from the Darwinian theory. Still 
some years passed after the appearance of Darwin's 
reforming book ere this rebellion was general. For 
Darwin very wisely did not, in his first book, deal with 
the most important consequence of his teaching, the 
Evolution of man from the lower animals. Further he 
had put on one side the question as to the first appear- 
ance of life. But very soon that consequence, so full 
of meaning, so wide-reaching, was openly discussed by 
able and brave scientific men, such as Huxley, Carl 
Yogt, Ludwig Biichner. A mechanical origin of the 
earliest living form was held as the necessary sequence 
to Darwin's teaching. Then the storm rose in its full 
strength — a storm whose fury will for some time to 
come throw into commotion the world of thought — a 
storm that will end in the victory of the truth of 

The same threats, the same fears as in the time of 
Copernicus and of Galileo, are again aroused by the im- 
perturbable advance of scientific knowledge. With 
those articles of faith denied by science, not only reli- 
gion but morality will perish. Whilst science is free- 
ing humanity, pining for liberty from the tyrannical 
chains of superstition and authoritative rule, it must 
throw into anarchy and ruin the whole order of the 
state and of society. But just as in the former time, 
in the sixteenth century, the new teaching as to the 
motion of the planets round the sun was the mighty 
lever that caused a great advance in the true knowledge 
of Nature and thereby in civilisation as a whole, so we 
hail Darwin's teaching as the morning star of a new 
period in the history of human culture, a period that 

D 2 


will as far surpass the to-day as this has the darkest 
time of the Middle Ages. 

During the six years that have elapsed since the 
issue of Darwin's work so many writings, large and 
small, have been published dealing with it that we may 
regard the principles of his teaching as generally 
known. We can here omit a full account of them the 
more readily as they have been already very thoroughly 
discussed in most of their bearings, and as we are at 
present only concerned with a single consequence of 
the theory, the natural origin of the human race through 
almighty Evolution. Still we are obliged, before entering 
upon this special inquiry, to say something on the 
groundwork of the Darwinian teaching itself and its 
essential connexion with the subject more immediately 
before us. 

It has been already stated by a number of notable 
writers, both adherents and opponents to the Darwinian 
theory, that the latter is so essentially connected with 
the notion of a progressive development of man from 
lower Vertebrates, that the one idea cannot be thought 
of without the other. This is a reflexion of the greatest 
moment. It may be that related groups of animals 
and of plants, as all the species of a genus, all birds or 
Cruciferae are descendants of one and the same origi- 
nal form, and have arisen from one common ancestral 
bird or Crucifer-form through transformations extend- 
ing over a very long time. In that case man also has, 
beyond a doubt, arisen from lower Mammalia, apes, the 
earlier simian creatures, the still earlier Marsupialia, 
Amphibia, Pisces, by progressive transformations. Or 
this is not the case, and the separate species of animals 
and plants have been independently created. Then 
man also has been created independently of other 
Mammalia. But if we believe in such a supernatural 
creation, we must take refuge in an incomprehensible 
marvel, and simultaneously give up all hope of a real 
understanding, a scientific explanation of the most 
important processes of Nature. If we can only prove 
the general truth of the Darwinian theory, our idea of 
the origin of man from lower Vertebrata follows of 
necessity, and we are not obliged to give a special proof 


as to this latter view if the general proposition is well 

Darwin's theory asserts, as is well known, that every 
resemblance that we observe in the general organisa- 
tion of the animals or plants of any natural group, 
e.g., of a family or a class, is a family relationship, de- 
pending on ties of blood, and that the word " affinity," 
by which we generally denote, figuratively, this 
similarity of structure, has in fact not merely a figu- 
rative, but an actual significance. The species related 
in form are, according to Darwin, related by blood. If 
this is true, the so-called " Natural System," in wiiich 
the scientific man ranks the various species according 
to the greater or less amount of their resemblance one 
to another, must be the veritable genealogical tree of 

On account of the extraordinary importance of this 
conception from the point of view of our lecture, we 
must illustrate it by an example. Let us take it from 
one of the familiar domestic animals, the cat. All the 
different kinds of cat are regarded by naturalists as 
offshoots of a single primordial ancestor, and are 
therefore placed in one species (Felts domestical) . But 
the genus Felis includes, besides the domestic cat, 
many other kinds, as the lion, the tiger, etc. All these 
different kinds of the genus Felis, or cat, agree in their 
bodily form, in the structure of their teeth and feet so 
completely, that we regard them in consequence as 
kinds or species of a single genus. In turn we find a 
common origin of all the different species of cats from 
one ancient, common ancestor. The lion {Felis leo), 
tiger (Felis tigris), Puma (Felis concolor), leopard 
(Felis leojjardus), wild cat (Felis catus), domestic cat 
(Felis domestica), are later offshoots in different direc- 
tions of that ancient, long -dead ancestral form. In 
like manner we consider the genera cat and hyaena, 
that are included in the family of cat-like beasts of 
prey (Felidae) as descendants of a single cat-like carni- 
vorous form, that lived at a stage of the Earth's history 
far earlier than that of the primordial Felis. In like 
manner the genera and species in the family of canine 
beasts of prey (Canidce), had originated from one dog- 



like ancestor, all the Ursidse from one bear-like, all 
the Mustelidse from one ancestor of a structure akin to 
that of the marten. 

If we now rise still higher in our study of the 
natural system of animals and compare all the last- 
named families, we see in all beasts of prey, in all 
feline, canine, ursine, marten-like animals such an 
agreement in the most important zoological marks, as 
in the form of the teeth and the feet, and so evident a 
difference between them and other animals, that we 
combine all these families into one great natural group,, 
the order Carnivora. If we are followers of Darwin we 
are by this combination expressing the genealogical 
- idea that all these carnivorous animals take origin from 
a single primordial form. It is obvious that this an- 
cestor of the whole order must be of far greater 
antiquity than his more recent descendants the indi- 
vidual parents of the families discussed above. 

As we can assume a common ancestor for all Carni- 
vora, the like principle holds for other orders of 
Mammalia, the Ungulata, the Quadrumana, Cetacea, 
the Marsupialia. All these different orders of the class 
Mammalia agree in the peculiarity of nourishing the 
newborn offspring by the milk of the mother, whence,, 
indeed, this class takes its name. Further, all Mam- 
malia agree one with another, and differ from all birds 
and other lower Vertebrata (Reptilia, Amphibia, Pisces) 
in many important characters of internal structure. 
For example, the lower jaw of Mammalia is much 
simpler in its construction than the lower jaw, com- 
posed of several distinct bones, of Aves and Reptilia. 
In addition, the lower jaw in the latter classes is 
connected with the skull by a special bone, the 
quadrate, wanting in Mammalia. [The quadrate 
bone of Aves is now known to be the homologue 
of the malleus, or first bone of the chain in the 
middle ear of Mammalia. — Trans.'] Moreover, the 
Aves and Reptilia have a nucleus in their red 
blood-corpuscles, and this structure is wanting in the 
Mammalia. In the latter class the skull is articulated 
with the first cervical vertebra by two condyles, in the 
former classes by one only. In these points, and in 


several others, all mammals, however different they 
may be one from another, on other points agree. They 
are more nearly related one to another than any mam- 
mal is to a bird or to a reptile. In like fashion all 
birds and all reptiles show many greater points of in- 
trinsic agreement than any bird shows to any reptile. 
The zoologist, therefore, notes all these points of re- 
semblance and of difference, and unites all mammalian 
orders in the one class of Mammalia, all bird orders in 
the class Aves, all reptilian orders in the class Reptilia. 
But behind this systematic statement we, with Darwin, 
behold the momentous truth that all Mammalia take 
origin from one common ancestral mammalian form, 
all Aves from an ancestral bird-form, all Reptilia from 
a common reptilian progenitor. 

As we pass upwards in this wise in the natural 
system of animals (and the same principle holds also 
among plants), we rise by degrees from the smaller, 
lower, younger groups, to larger, higher, older ones, i.e., 
the originals of the former. We pass thus from species 
to genera, from genera to families, from these to orders, 
and from orders to classes. Every higher group is made 
up of several lower and subordinate groups. Every 
higher group is on our genealogical idea of the natural 
system an elder branch of the ancestral tree, and the 
lower groups comprised in it are the younger boughs 
and twigs of that branch. If the teaching of Lamarck 
and Darwin as to the origin of living beings is rightly 
understood, all the plants or animals that we place in 
one class are without a doubt the offspring or de- 
scendants of one single common ancestral form. But 
we may go at least one step further, and finally with 
tolerable certainty affirm a common origin for all those 
classes of animals and of plants that are so alike in all 
essential marks of their organisation that naturalists 
have, since the beginning of our century, after the re- 
searches of Bar and Cuvier, grouped them around a 
so-called central type. 

Such a type, more accurately called a stem or phylum, 
is that of the Vertebrata, to which the classes Mam- 
malia, Aves, Reptilia, Amphibia, Pisces belong. The 
Mollusca constitute a second type, with the classes 



Cephalopoda, Gastropoda, Lamellibranehiata and Bra- 
chiopoda. A third phylum consists of the classes 
Insecta, Arachnida, Myriapoda, Crustacea : it is that of 
the Arthropoda. In each of these three types the 
general structure and the individual plan of develop- 
ment are so typical and characteristic that upon these 
grounds we can assert with certainty the blood-rela- 
tionship of all the members of the type. All the diffe- 
rent Yertebrata must have originated from one common 
ancestor, a single primordial vertebrate animal ; all the 
Mollusca from a primordial soft-bodied animal ; all the 
Arthropoda from a primordial articulate animal. 

The facts of comparative anatomy and development 
that prove, beyond a doubt, these blood-relationships of 
all the animals of one stem or phylum or type, are so 
convincing to the student of these matters that he can 
cite no stronger proofs than they of the truth of the 
teaching as to the origin of living things. Turning 
specially to the Yertebrata that are at present the most 
interesting of all to us, we find that they present an 
agreement throughout all the special structure and 
arrangement of their skeleton and nervous system that 
is evident in no other group of animals. The internal 
skeleton of Yertebrata consists in all cases at first of a 
central axis, a cartilaginous rod, generally replaced 
later on by bone, named the notochord or chorda dor- 
salis. From this the vertebral column is developed. 
From the dorsal surface of this chorda dorsalis two 
arched processes grow upwards towards what is to be 
the back of the animal. These unite to form a closed 
tube, and within this tube the most essential part of 
the nervous system is enclosed, the spinal cord, that all 
Yertebrata, without any exception, possess, and that is 
wanting in all other animals. Below the notochord 
lies the body cavity, enclosing the alimentary canal 
and its appendages, the lungs, the liver, and other 
organs. From these anatomical details, quite apart 
from the similar facts that could be given in confirma- 
tion of them from the history of development, a common 
origin of all vertebrata can be predicated with the 
greatest certainty if the teaching of Darwin is correct 
in its general principle. 


The classes of the animal kingdom that are still left 
for consideration after the three types of Vertebrata, 
Mollusca, and Arthropoda, were united by Bar and 
Cuvier into a fourth and last type, the Radiata. This 
phylum is not like the three previous ones, natural. It 
is an artificial collection of several very different stems 
or phyla. In the present condition of our zoological 
knowledge this group of Radiata must be replaced by 
at least four different phyla named as follows : (1), 
Echinodermata : the four groups Asteridae, Crinoideae, 
Echinidae, Holothuridae. (2), Vermes, or worms : the 
many forms of true Vermes in the most recent zoolo- 
gical sense, e.g., Tseniada, Nematoidea, Trematoda, 
Tunicata, Annelida. (3), Coelenterata : zoophytes, plant- 
like animals, the three classes of Spongida, Actinozoa, 
Hydrozoa ; and lastly (4), Protozoa : the Rhizopoda, 
Myxomycetes, Flagellata, Protoplasta, and many other 
organisms of the lowest rank, down to the lowest of 
all, the Monera. 

Amongst these four lower animal types the two phyla 
of the Echinodermata and the Coelenterata are groups 
of species related by blood, as natural as the three 
higher phyla. This can be said with little certainty of 
the Vermes, and with still less of the Protozoa. The 
group Vermes includes very different forms, and 
amongst these are the primordial forms of the higher 
animal types. The Vertebrata are placed in genealo- 
gical connexion with the Vermes through the Tunicata, 
the Mollusca through the Polyzoa, the Arthropoda and 
Echinodermata through the Annelida and the radiate 
worms. On the other hand, the Coelenterata are also 
connected with the Vermes. Lastly, the division Pro- 
tozoa, whose position is still very uncertain, includes, 
first, the original progenitors of the Vermes and Coelen- 
terata, and, second, a great number of organisms lowly 
and imperfect, that are neither genuine animals nor 
genuine plants. We shall do most wisely in placing 
these last in a special neutral group, midway between 
the vegetable and the animal kingdoms — the Protista. 
The systematic relationships of all these groups of or- 
ganisms can only be explained and comprehended by 
the teaching of Evolution. 



The natural system of plants and animals, as it has 
been taught this long while by zoologists and botanists, 
does not merely therefore serve the purpose of arrang- 
ing the different forms in many groups placed side by 
side, or in series according to the greater or less extent 
of their resemblance, and thus of facilitating the study 
of their endless variety of forms. The sole aim of the 
natural system of organisms is not simply a grouping 
of our anatomical knowledge as to the structures of 
living things. The natural system has a meaning far 
higher, far more widely-reaching than this. It reveals 
to us the facts as to blood relationships in organisms. 
It reveals their veritable and actual genealogy. 

At present the theory of descent that explains in 
this manner the natural system of organisms as their 
genealogical tree is generally associated exclusively 
with the name of Darwin. Yet historical truth requires 
the confession that before Darwin many naturalists 
had arrived at the same fundamental conception, and 
had in part worked it out. Notably at the beginning 
of our century the philosophers, with our great poet 
Wolfgang Goethe and the immortal Lorenz Oken at 
their head in Germany, with Jean Lamarck and 
Geoffroy St. Hilaire as leaders in France, guided by 
their investigations in comparative anatomy, asserted 
a common origin for related forms of animals. Goethe, 
as early as 1796, gave forth the remarkable utterance : 
" This also we had gained. We were able unreservedly 
to affirm that the more perfect organic natures, under 
which we include Pisces, Amphibia, Aves, Mammalia, 
and at the head of these last, Man, have all been formed 
on one original plan that is only slightly modified in 
its very constant details here and there, and is still 
improving and altering generation after generation. ,r 
And in another place Goethe says (1824) : "An in- 
ternal and original connexion runs through all organ- 
isms. Their different forms arise from their necessary 
relations to the external world, and we ought, there- 
fore, to accept as facts original and simultaneous 
distinctions, and a continually advancing transforma- 
tion, that we may understand the phaenomena of the 
organic world, both those that are constant and those 


that vary." In these and in other words of Goethe the 
foundations of the theory of descent — named by others, 
the theory of modifications or of transmutations — are 
recognisable. Nevertheless, the honor of having pub- 
lished these exceedingly important ideas for the first 
time as a scientific theory, at once well-founded and 
thoroughly thought out, belongs to Lamarck, whose 
" Philosophie Zoologique," issued in 1809, we may 
place side by side with that revolutionary teaching of 
Copernicus in its power of opening out a new path for 
human thought. 

One would think that the theory of descent, which 
threw at once full, clear light on the Evolution of 
organic species that had been until that time involved 
in ignorance and obscurity, would have wrought im- 
mediately upon its recognition a revolution similar to 
that of Copernicus in the whole of scientific inquiry. 
But this was not the case. On the contrary, the theory 
of descent that lays an indispensable and luminous 
foundation for all scientific Zoology and Botany was so 
little noticed in the first half of our century that even 
in the fourth and fifth decade it seemed almost for- 
gotten. This is the result chiefly of the want of a 
unified comparative study of the whole of organic 
Nature, and of the exclusive absorption in the close 
study of details that distinguished the naturalist of 
that age. On the other hand, the opposition of weighty 
authority placed great difficulties in the way of the 
extension of the new idea, and the two distinct sets of 
thinkers, the zoologists and botanists, isolated and 
working separately, failed to see clearly the necessity 
of allying their forces under the banner of the harmo- 
nising principle of the theory of descent. 

The inestimable service of Charles Darwin, whose 
work on the " Origin of Species," appearing in 1859, 
awoke on a sudden the moribund theory of descent to 
a new forceful life, lies not simply in his presenting 
that theory in a far more comprehensive and perfect 
form than all his predecessors, nor in his having 
strengthened it by all the proofs gathered together 
under the various branches of biological science. 
Another, a greater service of the renowned English 


naturalist, consists in this : that he enunciated for the 
first time a theory that explained mechanically the 
fact of the origin of species. He traced that origin 
back to physical and chemical causes, to a force of 
Nature working blindly, unconsciously, without design. 
This theory, crowning and completing the whole 
edifice of a mechanical comprehension of Nature is the 
theory of Natural Selection. It may be spoken of, in 
brief, as the theory of selection. This is the veritable 
Darwinism. It is not accurate to comprehend under 
this name the general teaching of Evolution or the 
theory of descent. If we would mark the latter by 
the name of its most prominent founder, we must call 
it Lamarckism. 

The blind, unconscious forces of Nature, working 
without end or aim, that Darwin showed to be the 
effective natural causes of all those complex forms of 
animal and plant life that are apparently built in 
conformity with a definite purpose, are the two pro- 
perties of living things known as heredity and 
variability. All organisms, all plants and animals, 
without exception, have these two momentous pro- 
perties. They are only special modifications of two 
other more general functions of living matter, repro- 
duction and nutrition. Variation is most intimately 
connected with the nutrition of the individual, heredity 
with the propagation of the organism. But as now all 
the phenomena of nutrition and of reproduction are 
purely natural processes, and are only the result of 
physical and chemical causes, so these latter are suf- 
ficient for those special phenomena that we call the 
functions of variability and heredity, important and 
mysterious in their working though they be. The 
reciprocal working of these two functions, and the 
special external circumstances under which they act 
and re-act one on the other, are the sole causes of 
organic structures and their modifications. Amongst 
these external circumstances, by far the most im- 
portant are the varying relations in which every 
organism stands to its organic environment, to the 
plants and animals that live in the same place with it. 
The totality of the varying conditions Darwin sums 


up under the name of the struggle for life. It has 
also been called u the fight for existence," " the com- 
petition for life," and best, perhaps, " the combat for 
the necessities of being." In a manner at once full of 
ingenuity, clear and convincing, Darwin shows that 
we can explain in simple fashion all organic structures, 
all the relations of form and of construction in organ- 
isms as the necessary consequences of the reciprocal 
working of variation and heredity in the struggle for 

As we cannot here enter more fully into the theory 
of Darwin, as a whole, we will only deal at length with 
these two principles that are so frequently wholly mis- 
understood. At the same time we may lead to a better 
understanding of the exceedingly important resem- 
blances and differences that are evident upon a com- 
parison of natural and artificial selection. By means 
of artificial selection the farmer and gardener can 
produce new organisms, even as Nature evolves them 
under natural selection. The new varieties of plants 
that the gardener, and even the new races of domestic 
animals that the farmer produces by artificial selection, 
are little different from the so-called kinds or species 
that the various animals and plants present to us in 
the wild condition. The commencement, and the 
method of their production, are the same in both cases, 
viz., the processes of selection or choice. Man makes 
use, by artificial systematic selection, of the two phe- 
nomena of heredity and variation. 

Whilst the construction and transformation of living 
forms takes place in like manner in artificial and in 
natural selection, and depends upon like causes, on the 
other hand real differences obtain between the two 
methods of selection. The reciprocal action between 
variability and heredity is conditioned and directed, 
in artificial selection, by the systematic working of 
man's will ; in natural selection, by the struggle for 
existence, working without end or aim. The changes, 
the new structures in plants and animals to which se- 
lection gives rise, serve in the artificial form for the 
use of the breeder, in the natural form for the use of 
the organism that is bred. Moreover artificial selec- 



tion produces, in a comparatively short time, new 
forms that differ from the original parent form in a 
very striking and remarkable way. Natural selection, 
on the other hand, works its transformations far more 
slowly, far more gradually. As a consequence, the 
changes in organic form that have been produced by 
artificial selection are much more transient, and easily 
disappear in succeeding generations, whilst the pro- 
ducts of natural selection are far more stable, and 
maintain their individuality through long series of 

Had Darwin not proved, in the perfect way in which 
he has, the origin of living things by his causative 
selection, had he not shown the variation of species to 
be the necessary consequence of Natural Selection, we 
should none the less be compelled to accept the teach- 
ing of Goethe and Lamarck on the subject, since theirs 
is the only theory that explains the totality of pheno- 
mena in organic nature. Those phaenomena that present 
themselves in the structural relationship between dif- 
ferent species of animals and plants, or in their so- 
called plan of structure, come specially within its ken ; 
and in addition, the facts of their geographical and 
topographical distribution, of their individual, and of 
their historic development, as they are revealed to us 
through the study of fossils or palaeontology. But 
most prominent of all is the notable, the very im- 
portant resemblance between the individual and the 
palaeontological development of organisms. All these 
and many other weighty truths are easily explicable 
on the principle of the theory of origins of Lamarck, 
on the conception that all the various species of plants 
and animals are the offspring of one or of a few very 
simple primordial forms that have undergone infinite 
modification, primordial forms that have arisen, not at 
the will or by the determinate action of a personal 
creator, but by archebiosis or spontaneous generation. 
All the common phaenomena that are known to us in 
the life of organic beings agree perfectly with this 
idea ; not a single fact contradicts it. Hence we 
are perfectly justified in placing the theory of de- 
scent as a broad and general inductive law at the 



head of organic science, at the head of Zoology and 

If, then, the theory of descent is in truth a necessary 
general inductive law, the application of it to man is a 
necessary special deductive law, a theory that follows 
unavoidably from the former. As the philosophical 
expressions, induction and deduction, on the right 
understanding of which everything in this connexion 
hinges, have been exceedingly misunderstood, an ex- 
ample may serve to explain them. At the time when 
Goethe was pursuing his studies in comparative ana- 
tomy, the want of an intermaxillary bone in man was 
regarded as the most important distinction between 
man and the rest of the Mammalia. The intermaxillary 
bone is placed in the median line of the mouth, 
between the two halves of the upper jaw, and carries 
the upperdncisors. As in all other mammals that had 
been examined as to this point, an intermaxillary had 
been found, Goethe drew thence the induction that 
this bone was common to all the class. Now as man 
does not differ essentially, in all structural particulars 
from Mammalia, Goethe arrived at the conclusion by 
deductive reasoning that man also must have an inter- 
maxillary. And, in fact, it fell to his lot to find that 
bone in the skull of man after careful investigation, 
and thus to furnish the actual proof of his deductive 
conclusion. In brief, a deduction is a conclusion, 
drawn from the general to the particular, an induction 
a conclusion drawn from the particular to the general. 

When from the agreement between all Vertebrata in 
form, structure, development, we draw the conclusion 
that all Vertebrata have arisen from a single common 
primordial form, our conclusion is an inductive one. 
But when we affirm a like origin for man, who resem- 
bles in every other particular of an essential nature, 
the Vertebrata, our conclusion is a deductive one. This 
reasoning from generals to particulars is the more 
sound and reliable, the greater the soundness and 
reliability of the inductive reasoning from particulars 
to generals on which it is founded. But if the latter 
rests upon the broadest inductive basis, we may regard 
the former as equally well founded. The greatest 



stress must be laid on this philosophical /proof as to 
the human genealogical tree. 

Many special facts have been brought to light that 
support the above deduction as to man by the extra- 
ordinary strides made during later years in the inquiry 
as to the early history and the age of the human race, 
and in the celebrated investigations into the stone, 
bronze, and iron ages. Similar facts have been the 
result of the modern science of Comparative Philology. 
Zoologists and geologists, antiquarians and historians, 
philologists and ethnographers join hands in strength- 
ening this theory of vast significance, in building it up 
in detailed particulars. Important and worthy of con- 
sideration as are all these contributions to the history 
of man, we can only see in them corroboration or 
verification of that deductive conclusion, that we have 
drawn with absolute certainty from the general in- 
ductive law of descent with modification. 

What means are in our possession for building up 
the zoological genealogy of the human race, in con- 
formity with the theory of descent ? The same means 
that we use for a like end in regard to other animals : 
above all the comparison of their external form, their 
internal structure, the history of their development. 
In the first place, we need only inquire into the place 
of man in the zoological system. For this system is 
nothing more than the simplest expression of the facts 
of blood-relationship as presented to us by the study of 
Comparative Anatomy, by the thoughtful comparison 
of external form and internal structure. On this point 
there is no doubt that man must be placed in the class 
of Mammalia, and that he belongs to that smaller group 
in this class that zoologists name Discoplacentalia or 
Mammalia with a placenta discoid in shape. This 
group contains five different main divisions of the 
rank of orders, namely the Rodentia, Insectivora, 
Cheiroptera, Prosimise, and Apes. It is evident that 
of these five orders man stands much nearer that of the 
apes than the other four. Therefore, now the question 
narrows itself down to this : whether man is to be 
placed in the ape order itself, or if he has the right to 
demand a special order to himself near the latter. 



However this subordinate question may be decided, 
this fact is certain, that amongst all animals the true 
apes, and especially the narrow-nosed apes of the old 
world, the so-called Catarrhini, approach more nearly 
to man than all other animals. Huxley, supported by 
the most evident discoveries in Comparative Anatomy, 
could utter the momentous sentence that the anatomi- 
cal differences between man and the highest apes are 
less than those between the latter and the lowest apes. 
In relation to our genealogical tree of man, the neces- 
sary conclusion follows that the human race has 
evolved gradually from the true apes. 

Whilst this very important fact was decided with 
enough of certainty by Comparative Anatomy, it re- 
ceived the most valuable confirmation from the study 
of Comparative Development. If we trace the evolu- 
tion of each human being or individual from the 
beginning of his life, we are not able, at first and for a 
long time, to demonstrate the least difference between 
man and the rest of the Mammalia. Like all the rest, 
€very man at the commencement of his life arises from 
a simple egg, a globular albuminous body only one- 
tenth of a line in diameter, surrounded by a delicate 
envelope, and containing a smaller globular mass, also 
of an albuminous nature, the germinal vesicle or 
nucleus of the egg. The human ovum, like that of 
every mammal and every animal, is a simple cell. 
This cell divides into two halves, and these again 
divide. By continuous division a mass of cells arises, 
from which the germ or embryo is formed. The latter 
has at first the form of a simple, circular disc that 
assumes later on a fiddle-shape, and consists of two 
concentric layers of cells. Very gradually all the dif- 
ferent parts and organs that make up the body of the 
growing mammal arise from this simple germ by a 
long series of changes, transformations and improve- 
ments ; up to a certain time the germs or embryos of 
all mammals, including man, are alike, and can only 
be distinguished by their size. Then by degrees dif- 
ferences, at first slight, soon more marked, appear that 
correspond with the systematic division of classes into 
orders, families, genera. In this connexion it is most 




noteworthy that the human embryo is not at all dif- 
ferent from that of the ape until a very late period in 
its life, long after the difference between the embryo 
of the ape and that of other mammals has been evi- 
dent. Only at a late period, towards the end of 
embryonic life, just before birth, are those differences 
recognisable that separate the true human embryo 
from that of the allied tailless apes. After birth, these 
points of difference are only trifling, and by degrees- 
acquire significance as man on the one hand, the ape 
on the other, is modified into his own definite in- 
dividual form. 

The life-history of the human being is, as the phy- 
siological laws of heredity and variation clearly show r r 
in its individual nature but a short, condensed repe- 
tition, a recapitulation, as it were, of the history of 
development of the animal type connected with man 
by blood, the vertebrate type. This history of types, 
or the so-called history of Evolution, is at present very 
little known to us ; for the actual witnesses on this 
point, the fossil remains of animals, have been found 
by man very sparingly on the whole. If we had to 
learn the history of man's ancestry from the fossils 
alone, we should be badly off. Yet these fragmentary 
proofs from ancient times are very valuable. From 
them we derive the foundations of the human pedigree 
in the different geological periods before man appeared 
on the earth. In the oldest system that has yielded 
vertebrate fossils, the Silurian, we find only remains 
of the lowest class, that of fishes. This is the predo- 
minant class throughout primary times. In the later 
periods only of the primary times, Amphibia accompany 
the fishes. These are the Vertebrata that evolved in 
closest relationship to the fishes. Later on, in far more 
recent strata, laid down during the secondary period, 
we meet the fossil remains of the three higher verte- 
brate classes, Reptilia, Aves, Mammalia. Of this last- 
class we only find throughout the secondary age the 
lower division of Marsupialia or Didelphia (kangaroo), 
but not a single trace of the higher division of placental 
mammals or Monodelphia. These latter, to which man 
belongs, make their first appearance at the beginning 



of the third great division of the earth's history, during 
the tertiary age. Further, we have in the successive 
series of vertebrate fossil remains during these three 
geological periods important evidence as to the original 
evolution of the human race, and the advancing de- 
velopment of the Vertebrata from the fishes upwards. 
Naturally this evolution required an enormous length 
of time. This lapse of time is proved by the thick- 
ness of the strata deposited from the water. We 
estimate the duration of these periods not by hundreds 
but by millions of years. 

Great as is the importance of the vertebrate fossils 
as the oldest records of man's ancestry, yet we should 
not be in a position to restore, by their aid alone, man's 
genealogical tree, as will be done in the next essay. 
Of the many thousand dead species of Vertebrata 
amongst which our ancestors existed, only a very few 
have been preserved, under fortunate circumstances, 
as fossils, and of these few only certain hard parts, 
specially adapted for preservation, as teeth and bones. 
But embryology, or the history of development of the 
individual, comes to our aid as the most reliable of 
allies. This stands in most intimate relation to palae- 
ontology, or the history of development of the type, as 
I have already shown. The succession of different 
forms through which every individual of any species 
passes from the beginning of its existence, from the 
ovum to the grave, is a condensed repetition of that 
series of different specific forms through which the 
ancestors and progenitors of this animal species have 
passed during enormously long periods of geological 
history. On the ground of this uncontradicted and plain 
evidence of embryology and palaeontology, on the 
ground of the complete parallelism of these two series 
in Evolution, on the ground, finally, of the whole testi- 
mony of Comparative Anatomy that is in accord with 
these, and of study of the geographical distribution of 
animals, we are in the position to assert, with full con- 
fidence, the evolution of the human race from the 
lower Vertebrata, immediately from the apes, more 
remotely from the Marsupialia, Amphibia, Fishes, etc., 
and to sketch the genealogical tree of man with ap- 



proximate certainty, as we shall try to sketch it in the 
following lecture. 

Science follows truth as her single aim. She can 
approach her goal only by the sure path of actual ex- 
periment and careful reasoning on that experiment, 
never by the false route of pretended revelation. It 
is to her all the same whether such conclusions, founded 
on the experience of the senses, be agreeable or anta- 
gonistic, welcome or repellant to the inclinations, 
wishes, and feelings of men. She treats, therefore, 
with indifference, the storm of anger and horror that 
has arisen against the discovery of the descent of man. 
Yet we cannot conceal our personal conviction that the 
fear that has been expressed as to this vast advance in 
knowledge by well-intentioned men is unfounded. 
The knowledge of man's origin, far from leading to 
deterioration, to degradation, leads to the improve- 
ment, the ennobling of the race. It hastens on the 
progress of man's mental development, of freedom in 
no measured fashion. 

We return at this point to the idea with which our 
lecture began, to the comparison of the theories of 
Copernicus and Newton with those of Lamarck and 
Darwin. By the system of Copernicus, the mechanical 
basis of which Newton founded, by the laws of gravity 
and of gravitation, the geocentric idea of the universe 
was overthrown, i.e.,, the false conception that the 
earth was the centre of the universe, and that the other 
bodies, sun, moon, stars, existed only for the purpose 
of sweeping round and round this world of ours. By 
the Evolution theory of Lamarck, the mechanical basis 
of which Darwin founded by the laws of heredity and 
variation, the anthropocentric idea of the universe was 
overthrown, i.e., the false conception that man was the 
centre of the life of earth, and that the rest of Nature, 
animals, plants, minerals, existed only for the purpose 
of serving him. 

The fears and accusations that were directed by the 
world in general against the system of Copernicus, and 
against the theory of gravitation of Newton, have been 
shown to be groundless and unjust. Instead of destroy- 
ing the moral order of the world, instead of over- 


whelming humanity in moral and intellectual ruin, 
these truths have raised morality and man to a higher 
standpoint as to the knowledge of what is ; they have 
purified and ennobled the world. They have rescued 
the peoples from the dark night of the sorrowful 
mediaeval times, and have lifted them into the morn- 
ing light of a new age. They have broken in frag- 
ments the bonds of ignorance, the fetters of supersti- 
tion, by which ambitious priests and princes strove to 
degrade their fellow-men to the level of blind tools of 
their arbitrary will. The tortures of the rack, of the 
Inquisition, by which a prejudiced priesthood sought 
to hinder and keep down the followers of the new 
truth, have but served to hasten their diffusion, to 
spread abroad the knowledge of them. 

The fate and the effect of Lamarck's theory of descent, 
and of Darwin's theory of selection, will, in many 
respects, be the same. But the views of these two 
men, and their application to man, aided by the mighty 
advances made of late in all branches of science, will 
gain a more rapid, a more universal victory, than the 
views of Copernicus and Newton, and their application 
to earth. Many favorable circumstances are combin- 
ing to clear the way for the theory of Evolution. Our 
whole conception of the universe has been altered by 
the colossal strides that have been taken in chemistry, 
physics, botany, zoology. By railroads and telegraph, 
our ideas of space and time have been revolutionised. 
By spectrum analysis and improved microscopes end- 
less paths of knowledge, once on a time undreamed of, 
have been opened. By all these giant strides in our 
advancing mental growth, we are prepared to grasp 
the greatest, the most pregnant discovery of all, the 
discovery of the natural origin, the animal descent of 
the human race. Potent in explanation, and, there- 
fore, powerful to ennoble man, it is at work every- 
where, and mankind moves more and more swiftly 
onwards towards its eternal goal, through the light of 
truth to the joy of liberty. 



IN the former lecture we arrived at the general con- 
clusion that the theory of descent must have its 
application in regard to man as well as to other orga- 
nisms. In this lecture we will try to solve the special 
problem as to what position in the genealogical tree of 
animals must be assigned to man. For the solving of 
this problem we use the same means as those by which 
we attained the general propositions of organic pedi- 
grees, viz., on the one hand the individual and palaeon- 
tological evolution, on the other Comparative Anatomy. 
The more completely two allied organisms agree in 
their embryological and palseontological development 
and in their anatomical structure, the closer is the 
affinity between them, the more nearly are they together 
in the genealogical tree. 

It has already been mentioned that we may regard 
all animals as descendants from six or seven types that 
correspond, on the whole, to the types of the animal 
kingdom originally distinguished by Bar and Cuvier. 
These were the types or phyla of the Vertebrata, Mol- 
lusca, Arthropoda, Echinoderma, Vermes, Zoophyta. 
The root common to and giving rise to these six 
animal types is to be sought for in the group of Pro- 
tozoa or Protista. It is clear that we can only picture 
this most ancient root of the higher animals as an 
organism of the simplest kind imaginable, as a struc- 
tureless, formless, minute particle of protoplasm, in a 
word, as a Moneron. The oldest Monera of this kind, 
simple living albuminous masses, without even the 
structure of a mere cell, can only have arisen through 
archebiosis or spontaneous generation. 

Of the six or seven phyla of the animal kingdom the 



phylum of the Vertebrata alone is at present of interest 
to us, as the human race is a branch of this stem. 
Until the present time four classes, Pisces, Amphibia, 
Aves, Mammalia, have been as a rule distinguished 
under the type of the Vertebrata. To the last of these 
man belongs. But if we now compare the different 
vertebrate groups genealogically, and try to construct, 
step' by step, their pedigree on the basis of the history 
of their development and of their Comparative Anatomy, 
we must distinguish the following eight classes : (1) 
Acrania ; (2) Monorrhini ; (3) Pisces ; (4) Dipneusta ; 
(5) Amphibia ; (6) Reptilia ; (7) Aves ; (8) Mammalia. 

The first class of the Vertebrata, the animals destitute 
of skull, or Acrania, is only represented by a single 
small animal so far below all other animals of this 
type that Pallas, its discoverer, regarded it as an imper- 
fect snail, minus a shell. This very remarkable animal 
lives in the sands of various seas, e.g., the Indian 
Ocean, the North Sea, the Mediterranean near Naples. 
It bears the name of Lancelet (Amphioxus lanceolaius). 
It has no head at all, no skull, no brain. All the rest 
of the Vertebrata have these and are named Craniata. 
A true heart, such as exists in others, is not present in 
this animal. The blood is circulated through the body 
by regular contractions of the blood-vessels. Hence 
the special class that Amphioxus constitutes may also 
be named Leptocardia, and in contrast with it all other 
Vertebrata that have a centralised heart may be called 
Pachycardia. The lancelet is very like a leaf, colorless 
or with a glimmer of red, half transparent, very small, 
lancet-shaped, about two inches long. But that this 
Amphioxus, despite the want of head, skull, brain, 
heart, is nevertheless a Vertebrate, is proved by its 
spinal cord and by the cartilaginous rod or chorda 
dorsalis that underlies the spinal marrow. These two 
highly important organs, spinal cord and chorda dor- 
salis, belong exclusively to the Vertebrata, and are 
wanting in all other animals with the sole exception of 
the Ascidians. The Ascidioida, the class to which the 
latter belong, are the nearest allies in blood of the 
Vertebrata. In man, as in the rest of the vertebrate 
sub-kingdom, the internal skeleton consists in the 



earliest period of embryonic life of this notochord 
alone, and the central nerve axis consists only of the 
spinal cord that lies above it. The brain and the skull 
that encloses it are developed later on by the differen- 
tiation of the anterior part of the notochord and its 
surroundings. Amphioxus exhibits throughout life in 
the structure of its most important organs the same 
lowest conditions of structure that all other Vertebrata 
run rapidly through in the earliest part of their em- 
bryonic life. It is clear that we have in this strange 
little animal the last surviving remnant of a low 
Vertebrate class that was well-marked at a very early 
period before the Silurian age, a class of which no 
fossil remains have come down to us on account of its 
want of enduring structures. Amongst these Acrania, 
the ancestors of the rest of the Vertebrata, the Craniata 
that later branched off from the former must have had 
their place. We must look upon Amphioxus with 
special reverence as the being that alone, among all 
the animals still living, is in a position to give us an 
approximate idea of our oldest Silurian vertebrate 

The second class of Vertebrata is far in advance of 
the Acrania, but is still so far below the Pisces that we 
cannot include it among these, as is generally done. 
To it belong the well-known lampreys (Petromyzon) 
that are so prized as a dainty, and the hag (Myxine), 
closely allied to these. Whilst in all other Vertebrata 
the nose consists of two similar lateral moieties, the 
nasal cavities, it consists in the Petromyzontes and 
Myxinoids of a single unpaired median part, and the 
whole class can be named Monorrhini, in contrast to 
the rest of the Craniata or Amphirrhini. Whilst the 
latter have three semi-circular canals in the labyrinth 
of the ear, only one or two are present in the Monorr- 
hini. The lower jaw and the peculiar sympathetic 
system that occur in all Amphirrhini are wanting in 
the lower class. By these and other peculiarities they 
rank far below the former, and in all probability we 
have to regard them as a solitary survival of an ancient 
vertebrate class once on a time very numerous, that 
constituted the bridge from the Acrania to the Am- 



phirrhini. The Acrania are the grandfathers, the 
Monorrhini are the fathers of the Amphirrhini. 

The third class of Yertebrata that commences the 
series of the Amphirrhini contains the true fishes, cold- 
blooded Vertebrata that breathe air dissolved in water 
by aid of gills. This class is divided into three sub- 
classes, the Selachii, the Ganoidei, the Teleostei. The 
first sub-class, that of the Selachii, or ancient fishes, 
comprises the sharks (Squali), rays (Rajse), and the 
Chimaerae that live in large numbers in certain seas. 
The second sub-class, that of the Ganoidei, or enamel- 
fishes, was very prevalent in the earlier ages of the 
earth's history, especially from the Devonian to the 
Jurassic, and constituted the chief part of the popula- 
tion of the seas of that time. Then it died out in great 
part, and was replaced in the Cretaceous time by its 
successors, the Teleostei. At the present time only a 
few survivors are in existence, such as the Polypterus 
of the African rivers, Lepidosteus and Amia in those 
of North America. But the most familiar Ganoids still 
living are the different species of the genus Accipenser, 
the Sturgeon and the Sterlet, whose eggs we eat as 
caviare, and whose swim-bladder yields us isinglass. 
Finally, the third sub-class of Pisces is the Teleostei, or 
osseous fishes, that have now far surpassed in numeri- 
cal development both the other sub-classes, but were 
first evolved from the Ganoidei in the Cretaceous or 
earliest of all in the Jurassic age. To this belong the 
majority of marine fish now in existence, and all fresh- 
water fish except the so-called enamel-fishes. 

The Comparative Anatomy and the history of the 
development of the three groups of Pisces enable us to 
determine their genealogy with the greatest certainty. 
The oldest division is clearly the Selachii, that were 
the first offshoot of the Monorrhini : and the most 
ancient fish in turn seem to be the sharks (Squali) that 
we on this account, and on account of their whole struc- 
ture, must regard as the progenitors of the rest. Further, 
the ancestors of man in the Silurian age must have 
been true sharks, or at least very closely allied to these. 
The sharks still in existence at the present time have 
altered very little since that period, far less than all 



other fishes, and especially than other Amphirrhini. 
In addition to this direct line of descent, with but slight 
modification, the ancient sharks of the Silurian age 
have left behind them descendants that have under- 
gone much modification. These are, on the one hand, 
the Ganoidei, from whom, later on, the osseous fish 
went off ; on the other, the Dipnoi, from whom, in all 
probability, the Amphibia arose at a later time. The 
Ganoidei, at all events, take origin from the Selachii, 
as the Teleostei, or osseous fishes, do from the Ganoidei. 
The Selachian branch may be named the grandfather, 
the Ganoid the father of the Teleostean. The oldest 
osseous fish, the Thrissopidae of the Jurassic age, from 
which all other bony fish have evolved, are most nearly 
allied to our sharks. Neither Ganoidei nor Teleostei 
can be regarded as direct ancestors of the higher Yer- 
tebrata : the Selachii alone can be thus regarded. 

As a fourth class of Vertebrata, we turn to the con- 
sideration of the Dipnoi, or mud-fish. These very im- 
portant animals are so exactly midway between the 
true fish and the Amphibia that the most famous zoo- 
logists are still at strife as to whether they ought to be 
placed among the former or the latter. This contest is 
best ended by placing these animals as a special class 
between Amphibia and Pisces. Of this intermediate 
group only very few remnants exist to-day. Some of 
these are in Australia (Ceratodus), some in the waters 
of the Amazon in South America (Lepidosiren), some 
in the African rivers (Protopterus). In winter, during 
the rainy season, the mud-fish live in the water and 
breathe by their gills air dissolved in water. In sum- 
mer, during the dry season, they build themselves a 
nest of leaves in the dried-up mud, and breathe air by 
their lungs. Their heart is conditioned as that of the 
Amphibia. Externally, on the other hand, they more 
resemble ordinary fishes. They are covered with scales 
like osseous fish. As the Dipnoi stand thus midway 
between Pisces and Amphibia, it is very probable that 
they connect these two classes in genealogical wise ; 
that they are descendants but slightly altered of those 
ancient Yertebrata that constitute the transition from 
the early piscine type to the Amphibia. 



The fifth class of Vertebrata consists of the true Am- 
phibia or Batrachia, in the limited sense in which this 
name is at present employed. From this class are ex- 
cluded the Dipnoi, just considered, and the Reptilia 
that were once regarded as akin to the Amphibia. 

To this class only the mail-clad Batrachia and the 
naked Batrachia belong. Of the former, the only 
living members are the small Csecilise, for the gigantic 
Labyrinthodonta of the Triassic age have long died 
out. To the latter belong the three orders of the gilled 
Batrachia or Perennibranchiata (e.g., the well-known 
Proteus of the Adelsburger Cave) ; the tailed Batrachia, 
or Urodela (salamander and water-salamander) ; and the 
Batrachians (frogs and toads). Of these three orders 
the Batrachia are the descendants of the Urodela, as 
these are of the Perennibranchiata. Every individual 
frog, every individual toad even now in its early meta- 
morphoses runs through these three stages, assuming 
at first the form of the perennibranchiate, then that of 
the tailed Amphibian, finally that of the adult frog 
without gills or tail. The Perennibranchiata, at all 
events, take origin from the earlier type of Pisces, 
either directly or through the mediation of the Dipnoi. 

The three remaining classes of Vertebrata, Reptilia, 
Aves, Mammalia, show a much closer relationship 
among themselves than to the preceding classes. At no 
time of their life do they breathe by gills, whilst in the 
preceding classes this is always the case, though it may 
only occur in the earlier stages of the animal's life. All 
Reptilia, Aves and Mammalia are, during embryonic 
life (so long as they are within the egg), surrounded by 
a special membranous, investment, the amnion, that is 
wholly wanting in the four classes already discussed. 
These and other facts show that the three classes of 
Reptilia, Aves, Mammalia, have evolved from a common 
ancestor, and that this latter has originated from an 
offshoot of the Amphibian group. Probably this com- 
mon ancestor of the three highest vertebrate classes 
divided early into two different lines. Along the one 
line Reptiles and Birds, along the other Mammals have 

The Reptiles will now be considered as the sixth 



vertebrate class, and the one that ranks next in suc- 
cession to the Amphibia. To this class belong lizards, 
snakes, crocodiles, turtles, and the great number of ex- 
traordinary, dragon-like monsters (Sauria) that were so 
largely developed in the secondary period of the 
earth's history, in the Triassic, Jurassic and Cretaceous 
ages, but died out completely at the end of that period. 
All these reptiles resemble externally the true Amphi- 
bia (Frogs, Salamanders, Perennibranchiata) and re- 
semble them also in their cold blood. But they are 
altogether different from the Amphibia in the most 
important particulars of their structure and in their 
development. They show rather a very striking kin- 
ship to Birds, with whom, as far as their external form 
and habits of life are concerned, they have but little 

The Birds (Aves) that rank next to the Reptilia as 
the seventh Vertebrate class have without doubt evolved 
from the latter class, and very probably from reptiles 
very nearly allied to the Dinosauria. This close affinity 
between Reptilia and Aves, which at first sight seems 
very unlikely, is established beyond dispute by the 
resemblance as to the most important characters of the 
organisation already mentioned, and by the whole 
development of the young in the egg. The class Aves 
is no other than a single branch of the reptilian group, 
that has acquired by adaptation to special life-con- 
ditions a number of special peculiarities of structure. 

The class Mammalia, eighth and last of the Verte- 
brate classes as distinguished by us, is the most 
important and the most highly-developed of all. At 
first sight it appears related most nearly to the Birds, 
with whom it shares, among other things, warm blood, 
complete separation of the right and left halves of the 
heart, the higher development of the brain and of the 
mental functions. But we are shown, by a series of 
important facts in the anatomical developmental 
history of the Mammalia, that this class has not 
evolved from the Birds nor from the Reptiles, but 
more probably directly from the Amphibia. It has 
already been said that we may certainly assume one 
common ancestral form for the three classes of Rep- 



tilia, Aves, Mammalia, a form that arose immediately 
from an offshoot of the class Amphibia. But the 
descendants of this ancestor, that lost altogether bran- 
chial respiration and developed an amnion, separated 
very soon, perhaps during or soon after the Carboni- 
ferous age, into two lines ; on the one hand the Reptilia, 
from whom the Birds sprang later ; on the other hand, 
the form intermediate between Amphibia and Mam- 
malia, whence finally the true Mammals took origin. 

Of all the classes of the animal kingdom that of the 
Mammalia is by far the most important and most in- 
teresting, if only on the ground that without doubt man 
— regarded by the unbiassed mind of the scientific 
thinker — must be placed in this class. Man has all the 
peculiarities and distinctive marks by which the 
Mammals are marked off from all other animals, and 
if the theory of descent is a general truth there 
cannot be the least doubt that the human race has 
arisen from this class by gradual evolution and differ- 
entiation. We must, therefore, of necessity, now give 
especial attention to the genealogy of this class, and to 
the systematic classification which is the expression of 
that genealogy. 

The older naturalists simply divided the class Mam- 
malia into a series of some ten to fifteen different 
orders. This series began with the order Cetacea, 
which seemed to claim the lowest rank by the fish-like 
shape of its body. It ended with the order of the apes, 
the Quadrumana that approached most nearly to the 
human form. From these, as a rule, the human race 
was separated, as the order Bimana. More recent 
zoology, that lays less stress upon external resem- 
blances than upon the far more significant differences 
in internal structure and development, has given, on 
the other hand, a very different classification of the 
Mammalian class. It marks off first three chief groups 
or sub-classes that are, despite their different extent, 
separated from one another so widely by their struc- 
ture and by the history of their development that they 
might even be regarded as distinct classes. These 
three sub-classes are the Monotremata, the Marsupialia, 
and the Placentalia. Probably these three groups are 



related one to the other as the Perennibranchiata, 
Urodela, and Batrachia among the Amphibia ; that is, the 
Monotremata group is the grandmother and the Mar- 
supialian the mother of the Placentalia. 

The first class of the Mammalia, that of the Ornitho- 
•delphia or Monotremata, is to-day represented by only 
two living genera — the one a water animal, the duck- 
billed Platypus (Ornithorhyncus paradoxus) ; the other 
a land animal Echidna hystrix. Both genera are con- 
fined to New Holland, that part of the earth that is the 
habitat of so many other classes of animals and of 
plants of simplest and most rudimentary organisation. 
These low forms are of deepest interest, for they tell 
us of that long-vanished time in which the higher and 
more perfect forms of the same class had not yet arisen 
from these lower organisms. We ought therefore to 
regard the strange Monotremata as the last survival of 
that most imperfect, lowest Mammalian group that 
began to evolve from the Amphibia at the end of the 
primary or at the beginning of the secondary period, 
the group from which later on the Marsupialia evolved 
as a more advanced offshoot. In all probability that 
group developed during the secondary period into a 
vast number of genera and species. But the strata of 
that long age only enclose for the most part remains of 
sea-dwelling organisms. No fossil remnants of the 
Monotremata that dwelt on land or were amphibious 
have come down to us. The Monotremata in their 
whole organisation, and particularly in certain im- 
portant features thereof, approach more nearly the 
lower Vertebrata, especially the Amphibia, than the 
higher Mammalia ; whilst, on the other hand, they pre- 
sent a number of marks in common with the Marsu- 
pialia that the Placentalia no longer possess. Hence 
the opinion arises that the Monotremata living to-day 
are but slightly altered lineal descendants of that 
ancient ancestor of the Mammalia that marked the 
transition from the Amphibia to the Marsupialia. The 
Monotremata therefore stand in the same relation to 
the rest of the Mammalia as the Pharyngobranchii 
(Amphioxus) to the rest of the Vertebrata. And in 
regard to the pedigree of man they have this special 




interest, that they bring before our eyes to-day a far-off 
picture of that lowest step in the Mammalian organisa- 
tion whence, in the beginning of the secondary period, 
our ancestors arose. 

The Didelphia, or Marsupialia, constitute the second 
sub-class of the Mammalia. They stand midway be- 
tween the first and the third sub-classes, between the 
Monotremata and the Placentalia, and probably form 
the connecting link between these, not only anatomi- 
cally but genealogically. The Marsupialia are children 
of the Monotremata, parents of the Placentalia. As 
well-known examples of this group I need only men- 
tion the Kangaroo (Halmaturus) and the Opossum 
(Didelphys), that are seen in every zoological garden. 
The Marsupialia have received their name from the 
fact that their young, born in a very undeveloped con- 
dition, are for a long time after birth carried about in 
a pouch of the mother until they are fully developed. 
The geographical distribution of this group is very 
limited. The majority of the Marsupialia still living 
inhabit New Holland and the adjacent islands. Only 
a very small number occur in the Sunda Islands and 
in America. But in the dim past, long before the appear- 
ance of the human race, they had a much wider range. 
Fossil remains of Marsupialia are found even in Europe. 
In their anatomy and in their development the Marsu- 
pialia rise considerably above the Monotremata, whilst 
they are still far below the Placentalia. Hence we 
conclude that they in turn form the transition between 
these two groups genealogically, as well as systemati- 
cally. It is clear that the placental Mammals, earlier 
or later (at the commencement of the tertiary period),, 
sprang from the Marsupialia in the same manner as in 
yet earlier time (at the commencement of the secondary 
period), the latter sprang from the Monotremata. This 
opinion is confirmed in striking fashion by palaeonto- 
logy. For all the fossil remains of Mammalia that we 
find during the long age of the secondary period 
(Triassic, Jurassic, Cretaceous strata) belong to the 
Marsupialia. But all the fossil remains of the Placen- 
talia known to us have been found in the strata laid 
down during the succeeding tertiary times. Hence we 



conclude, with tolerable certainty, that the Placentalia 
evolved from the Marsupialia early in the beginning of 
the tertiary or at the end of the secondary period. The 
ancient forerunners of the human race, during the 
secondary age, belonged at all events to the Marsu- 
pialia, even if they presented many important external 
points of difference from the kangaroo and opossum of 

The third and last sub-class of the Mammalia, the 
placental Mammals (Monodelphia or Placentalia) in- 
cludes all Mammalia except the Monotremata and 
Marsupialia. This is by far the most extensive of all 
the three sub-classes. It is also the most important 
for us, as it includes Man. This class receives its name 
from a special and very important organ that marks it 
off from the Marsupialia as well as from the Monotre- 
mata. This organ bears the name of placenta or after- 
birth. It is a spongy white and red body of varying 
shape, consisting in the main of very convoluted and 
peculiarly arranged blood-vessels. Its function is to 
nourish and to bring the maternal blood to the young 
placental animal during the time before birth whilst it 
is within the mother. 

The varying structure and external form of this; 
organ are very characteristic in the different groups or 
orders of placental Mammals. These can be divided 
into three different legions, each comprising a group 
of orders. These three legions, expressing three dif- 
ferent twigs of that branch of the genealogical tree that 
constitutes the Placentalia, bear the names Yilliplacen- 
talia, Zonoplacentalia, Discoplacentalia. In the first 
legion the placenta is composed of many separate scat- 
tered tufts ; in the second legion it is girdle-shaped, in 
the third discoid. 

The legion of the Villiplacentalia, or the animals 
with cotyledonary placentas, includes three orders, the 
Edentata, Ungulata, Cetacea. To the Edentata, an 
order far more strongly represented in the diluvial 
past than to-day, belong the ant-eater, armadillo, sloth, 
and their allies the gigantic Macrotherium, Megathe- 
rium, Mylodon, Glyptodon, etc., of the tertiary age. 
The order Ungulata is generally sub-divided into three 

F 2 



groups, the single-hoofed animals or horses (Solidun- 
gula) ; those with two hoofs, the ruminating animals 
(Ruminantia) ; and lastly those with more than two 
hoofs, the thick-skinned animals (Pachydermata). To 
this last the pig, rhinoceros, hippopotamus, and others 
belong. In the present day these three sub-orders of 
Ungulata appear in truth as distinct and clearly marked 
off one from another. But as soon as they are compared 
with their dead and gone ancestors of tertiary times, 
many of whose fossil remains are known to us, it be- 
comes clear that the three sub-orders are closely inter- 
connected by a series of intermediate transitional forms 
that have perished. "We may therefore conclude that 
all Ungulata have sprung from one stem, that the three 
sub-orders now existent are but three individual 
branches of that common stem. The third order of 
the Villiplacentalia (that of the Cetacea), is very 
nearly allied to the Ungulata. To this the whale, dol- 
phin, sea-hog, porpoise, sea-cow, belong. These marine 
animals only resemble fishes externally. By the whole 
of their internal structure, and by their development, 
they give clear evidence that they are Mammals, and 
that they are placental Mammals most nearly allied to 
the Uugulata. We are entitled, on fairly certain 
grounds, to hold that the Cetacea have taken origin 
from the Ungulata, that they are allies of the Ungu- 
lata which have become adapted for an aquatic life, 
and have in consequence taken on a fish-like structure. 
All Cetacea, Ungulata, Edentata, agree in that their 
placenta consists of many separate tufts, and in that 
respect, as in the constant absence of a decidua, it is 
essentially different from that of the Zonoplacentalia 
and Discoplacentalia. In the two latter divisions the 
placenta is always single and simple, and a decidua is 
always present. 

The legion of the Zonoplacentalia, in whom the 
placenta takes the form of a ring-like enclosing girdle, 
includes the Carnaria alone. These animals would 
seem, from the characteristic structure of their teeth 
and brain also, to be a special natural group, all of 
whose members are related by descent. They present 
two orders : the land beasts of prey (Carnivora), and 



those that inhabit the sea (Pinnipedia). To the latter 
the seal, sea-bear, sea-lion, walrus, belong ; to the 
former, cats, dogs, martens, badgers, bears, and many 
others. These two orders are as like one to the other 
as the Ungulata and Cetacea. Externally, again, the 
land and water carnivora are very unlike. But all 
their internal structure and their development prove 
to us, beyond a doubt, that they are very closely related 
in blood, and that the Pinnipedia have become so 
diverse from the Carnaria, their ancestors, only 
through adaptation to aquatic life. The habit of 
dwelling in the water, and the continual swimming 
therein — these alone have, under the influence of 
Natural Selection, fashioned some of the Carnivora 
into Pinnipedia, some of the Ungulata into Cetacea. 
Moreover, animals intermediate to the land and water 
forms of both groups are even now known ; among 
the Ungulata, the hippopotamus ; among the Carni- 
vora, the otter (Lutra), and yet more notably the sea- 
otter (Enhydris). 

The extensive legion of the Discoplacentalia, third 
and last of the three legions of the Placentalia, is the 
largest, most important of all ; for to this legion 
belongs the human race, and from its lower forms that 
race has been developed. The placenta of man has 
exactly the same shape and structure as that of all apes, 
monkeys, bats, insect-eating and rodent animals. On 
this ground, then, we cannot separate the genus Homo 
from other Discoplacentalia. In all these animals the 
placenta has the shape of a simple round disc or cake, 
and this shape occurs in no other living beings. In 
their possession of a desidua the Discoplacentalia are 
closely allied to the Zonoplacentalia, so that these two 
groups appear to be more closely related one to another 
than they are to the Sparsiplacentalia, or Mammalia 
with a diffuse and non-deciduate placenta. 

Usually the Discoplacentalia are divided into five 
orders : (1) Rodentia — squirrels, mice, porcupines, 
hares, etc. ; (2) Insectivora — shrewmouse, mole, hedge- 
hog ; (3) Cheiroptera — insectivorous bats, or Nysterida, 
and frugivorous bats, or Pterotocyna ; (4) Quadrumana 
— Prosimiae, Apes or Simise ; (5) Bimana — Man. 



The first three of the five Discoplacental orders, the 
Rodentia, Insectivora and Cheiroptera, we can leave 
unaltered in the position next one to another hitherto 
accorded them. But the Discoplacentalia of the fourth 
and fifth orders must be arranged in another manner. 
First of all, we must separate the lemurs (Prosimiae) as 
a special order from the true apes (Simiae). The former 
are very remarkable and important. In the earlier 
tertiary times, in all probability, many genera and 
species of Prosimiae lived. But at the present day this 
order is represented by a few living forms which have 
withdrawn into the wildest regions of Africa and Asia, 
into Senegambia and Madagascar, Upper India and the 
Sunda islands, and in these wildernesses live for the 
most part a nocturnal life. The different genera of the 
Prosimiae present very striking transitional forms, con- 
necting that sub-order with other Discoplacental orders. 
Thus the Chiromys of Madagascar is related to the 
Rodentia, the Otolicnus and Tarsius to the Insectivora, 
the Galeopithecus of Sundaisland to the Cheiroptera, 
and finally the Loris (Stenops), Indris (Lichanotus), and 
Makis (Lemur) to the true Simiae. Upon these and 
upon other grounds we are compelled to regard the 
still living Prosimiae as the last remnant of an ancient 
ancestral group, for the most part long perished, 
whence by evolution in different directions the other 
four orders of the Discoplacentalia have branched off. 
The parent forms of the Rodentia, Insectivora, Cheirop- 
tera and Simiae may therefore be regarded in a certain 
sense as four children of the same family having their 
common origin, their mother, in the Prosimiae. 

Whilst on the one hand we seem by our separation 
of the Prosimiae and Simiae to increase the number of 
the five Discoplacental orders by one, we reduce the 
number on the other hand to five once more, by uniting 
the order containing man alone (Bimana) with the 
order containing the true apes (Simiae). The celebrated 
English zoologist, Huxley, has shown, for the first 
time, in his remarkable " Evidences as to Man's Place 
in Nature," that we can no longer regard these two 
orders as distinct one from the other. For the Simiae 
have, like man, two hands in front, two feet behind, 



;and it was a blunder in anatomy when, in earlier 
times, the Simiae were credited with four hands, and 
their feet, as if opposed in nature to those of man, were 
named hands. In addition to this, we have the much 
more important fact that the most recent^investigations 
of all the special anatomical peculiarities of man and 
of the true apes have led Huxley to the following con- 
clusion : " The anatomical differences that separate 
man from the highest apes (gorilla and chimpanzee) 
are not so great as those that separate the highest apes 
from the lower." In fact, considering any part of the 
body, we may, from the most recent investigations, con- 
clude that man is more nearly allied to the highest 
apes than are these to the lowest. Hence it would be 
altogether forced and unnatural if man were separated 
from the apes as a special order in the zoological 
system. Rather is scientific zoology compelled, willy- 
nilly, to place man in the order of the true apes 
(Simiae). Then we recognise springing from the 
Prosimiae as the common ancestral group the following 
five orders of Discoplacentalia : (1) Prosimiae ; (2) 
Rodentia ; (3) Insectivora ; (4) Cheiroptera ; (5) 
Simiae (including man). 

If now we again call to mind that the natural system 
of animals is no other than their genealogical tree, we 
come to the conclusion that the human race have to 
seek their ancestors among the Discoplacentalia, first 
among the true apes, yet farther back among the 
Prosimiae. Shocking and impossible as this truth may 
seem to the majority of men, it can no longer bb 
doubted at the present time. Nay, zoology is even in 
a position to define directly this important offshoot of 
the human genealogical tree with more precision and 
oertainty than is possible in many other cases. With 
this end we must enter somewhat more at length into 
the systematic arrangement of the Quadrumana. 

The division of the true apes or Simiae is to-day 
arranged in two sub-orders, the Platyrrhini and Catarr- 
hini. The group Platyrrhini includes all the apes of 
the new world (America), amongst others the howlers, 
the weepers, capuchins, and squirrel-monkeys. The 
group Catarrhini, on the other hand, includes all 



the apes of the old world (Asia and Africa). To it 
belong the tailed baboons, the macaques, and, most 
important from our point of view, the remarkable 
family of the tailless anthropoid apes ; the gibbon 
(Hylobates), orang (Satyrus) in India, the chimpanzee 
(Pongo troglodytes) and gorilla (Pongo gorilla) in 
tropical Africa. 

The Platyrrhini of America and tne Catarrhini of 
Asia agree in many important points. For example, 
in both groups all the fingers and toes are provided 
with nails as in man, and not with claws as in the 
monkeys. But on the other hand, the two sub-orders 
exhibit many characteristic differences, especially in 
the structure of the teeth and of the nose. In all old- 
world apes the two nasal apertures look, as in man,, 
downwards, and the vertical partition that separates 
them is narrow and thin ; hence their name Catarr- 
hini. On the other hand, the nasal partition in all the 
new-world apes is broad and largely thickened below,, 
so that the two apertures do not look downwards, but 
sideways and outwards ; hence the name Platyrrhini. 
The apes of the old world resemble man also in their 
teeth as well as the construction of the nose. They 
have thirty-two teeth, viz., in each jaw, in upper as 
well as in lower, four incisors, two canines, four pre- 
molars and molars. On the other hand, the apes of the 
new world have thirty-six teeth. They have an addi- 
tional molar in each jaw on both sides. These anato- 
mical distinctions show plainly that the American apes 
have developed on their own continent independently 
of the old-world apes. Yet it is probable that the 
ancestor of the American ape sprang from the Asiatic,, 
and wandered over from Asia to America ; or perhaps 
the converse of these propositions is the truth. 

In all the anatomical relations mentioned above, man 
is exactly like the old-world apes, and it may further be 
laid down as assured that he is actually descended 
from these. The most detailed and exact investiga- 
tions of late years, viz., those of Huxley, have proved 
to demonstration that all the differences of form sepa- 
rating man from the anthropoid apes (the gorilla, 
chimpanzee, orang), are less than those differences 



(especially as regards the structures of the limbs and 
of the skull) which separate the highest known tailless 
apes from the lower tailed apes or baboons. If, there- 
fore, all the apes of the old world, from the lowest 
baboon to the most highly-developed gorilla, are, as is 
usually the case, included in one and the same group, 
the Catarrhini, it is not possible to exclude man from 
this same group. As to the genealogical tree of the 
human race, the inevitable conclusion is that we must 
seek our nearest brute ancestors among the Catarrhini. 
It is self-evident that none of the apes now living can 
be regarded as these ancestors. More probably these 
last have perished long since, and at the present time 
a gap separates man from the gorilla wide as that be- 
tween the baboon and the gorilla. But in this fact 
there is not the slightest evidence against the well- 
grounded conclusion that the most ancient Catarrhini 
form of the Prosimias was the ancestor common to all 
other Catarrhini, including man. It was a branch still 
unknown, and certainly long dead, of this extensive 
catarrhine group, that under favorable circumstances* 
by Natural Selection, evolved the ancestor of the human 
race. At all events, this transformation was of vast 
duration, and as yet neither place nor time has yielded 
us any fossil apes. But in all probability it occurred 
in Southern Asia, in which region many evidences are 
forthcoming that here was the original home of the dif- 
ferent species of man. Probably Southern Asia itself 
was not the earliest cradle of the human race ; but 
Lemuria, a continent that lay to the south of Asia, and 
sank later on beneath the surface of the Indian Ocean. 
The period during which the evolution of the anthro- 
poid apes into ape-like men took place was probably 
the last part of the tertiary period, the Pliocene age* 
and perhaps the Miocene age, its forerunner. 

As we look upon the apes that inhabit the earth 
to-day, and among other Yertebrata of the present time* 
we see but few races that are unaltered descendants 
of those Yertebrata that, according to the genealogical 
tree now in construction, we have to regard as ances- 
tors of the human race. In like manner, we are hardly 
able to point out with certainty, amongst the many 



fossil remains of Yertebrata that we find in the layers 
of the earth's crust, individual species as forerunners 
of the human race. Nevertheless we are in a position 
to state, with at least some certainty, the approximate 
ancestral series of man in the Vertebrate system gene- 
rally, the system which reveals to us in outlines the 
natural genealogical tree of the Yertebrata. 

The attempt that is here made, for the first time, I 
have worked out more fully in my " General Morpho- 
logy " (1866), and later in my " Natural History of 
Creation " (1868). 

Regarded as a whole, the series of the animal proge- 
nitors, or ancestral chain of man, can be brought under 
two groups, of which one includes the Yertebrata only, 
the other all those invertebrate animals out of whose 
gradual transformation and advancement the parent 
of the Yertebrata first originated. We may designate 
these invertebrate ancestors of the Yertebrata and of 
man as Prochordata. 

Until recently we could only make guesses very un- 
certain and vague in regard to these Prochordata. 
Suddenly the deep darkness of our invertebrate gene- 
alogy was lightened by an unexpected discovery of the 
greatest value. From the observations on the develop- 
ment of the lancelet (Amphioxus) and the simple 
Ascidioida (Ascidia, Phallusia), published by Kowa- 
lewski in 1867, the extraordinary and significant fact 
was ascertained that the ontogeny of these two beings, 
apparently so different, agreed in the most remarkable 
manner. The invertebrate Ascidioida are Yermes of 
the old class Tunicata, hitherto inaccurately referred to 
the Mollusca. In their adult state the Ascidioida are 
shapeless little masses affixed to the sea bottom, in 
which superficial observation would scarcely imagine 
that an animal existed. But these dull, inert masses 
arise by a retrograde metamorphosis from free-swim- 
ming active larvae, and these larvae develop after the 
same fashion as the lowest vertebrate, the Amphioxus. 
They possess, in fact, a rudiment of a spinal cord and 
of the chorda dorsalis that lies between the cord and 
the intestine. But these are the most characteristic 
and special parts of the body of Yertebrata. Hence 



it may be concluded with absolute certainty that the 
Ascidioida have, of all the Invertebrata, the closest 
blood-relationship to the Yertebrata. 

The Ascidioida, like the rest of the Vermes, have 
evolved, in all probability, from lower, more primitive 
forms, nearly allied to the Turbellaria and Gastroeada 
of to-day. We must regard as forefathers of these last 
such very simple, unicellular animals as the Amoebae, 
that are found in all water at the present time. That 
the earliest ancestors of the human race were such 
simple, primitive animals, whose whole structure was 
a, single cell, is strongly confirmed by the incontro- 
vertible fact that every human individual is developed 
from an ovum ; and this ovum is, like the ovum of all 
other animals, a single cell. Here, at once, the intimate 
causal connexion between the individual development 
of the particular organism and the historical develop- 
ment of its race leaps into view, and the simple con- 
nexion of ontogeny and phylogeny becomes of greatest 
significance. If, therefore, people find our theory of 
the origin of the human race "abominable, revolu- 
tionary, immoral," they must, in like manner, regard 
as " abominable, revolutionary, immoral," the facts, 
certain, demonstrable by a glance through the mi- 
croscope, that the human ovum is a simple cell, 
that this cell is in no wise different from the 
egg of other Mammalia, and that in the former, 
as in the latter, a multicellular body develops 
that repeats in brief, during the course of its embry- 
onic growth, every link in the ancestral chain of the 
Mammalia in the most important particulars. In this 
ancestral chain, or series of forefathers, we can, in the 
present state of our knowledge, mark off approximately 
twenty-two steps, of which eight belong to the Inver- 
tebrata, fourteen to the Vertebrata. 

Ancestral Series of Man. 

First Division of the Chain of Ancestors of the Human 

Invertebrate progenitors of man (Prochordata) : — 
First stage. — Monera. Organisms of the simplest 



structure conceivable, like the present Protamoeba,. 
Protogenes, Bathybius, made up of a shapeless minute 
mass of living protoplasm. The earliest Monera, from 
which later on the first cells evolved, can only have 
arisen by evolution from inorganic matter. 

Second step. — Amoebae. Organisms consisting of a 
simple, naked cell, made up of a shapeless mass of 
living protoplasm, and an enclosed nucleus. Probably 
these unicellular primal animals were not very differ- 
ent from the Amoebae of to-day, as the human egg is 
not essentially different from an Amoeba surrounded 
by a membrane. 

Third stage. — Synamoebae or colonies of Amoebae, con- 
sisting of a collection of similar naked cells, like the 
Labyrinthula of to-day, or like the morula stage of the 
fertilised egg. 

Fourth stage. — Ciliata or Planaeada, like the ciliated 
larva or blastula of Amphioxus and many invertebrates. 
Multicellular, hollow organisms, whose surface is beset 
with vibratile cilia. 

Fifth Stage. — Gastroeada, like the Gastrula stage of 
Amphioxus, evolved from the Planaeada by the forma- 
tion of a mouth and internal cavity. 

Sixth stage. — Turbellaria, or at all events certain low 
Vermes, at present unknown, of very simple structure, 
that are the next stage in complexity to the Gastroeada, 
and are most closely allied to the Turbellaria of the 
Vermes known to-day. 

Seventh stage. — Scolecida, forming the transition 
between the Turbellaria of the sixth and the Himatega 
of the eighth stage. 

Eighth stage. — Himatega or sack-worms, standing 
nearest to the Ascidioida, and especially to the Asci- 
dians among known animals. Like these they had 
evolved a rudiment of the spinal cord, and the sub- 
jacent chorda dorsal is. 

Second Division. 
Vertebrate progenitors of man (Vertebrata) : — 

Ninth stage. — Acrania. Vertebrata destitute of head r 
skull and brain, central heart, jaws, limbs, allied to the 
lancelet or Amphioxus of to-day. 



Tenth stage. — Monorrhini. Vertebrata with head, 
skull, brain, central heart ; without a sympathetic 
system, jaws, limbs ; with a simple single nasal cavity. 
Allied to the Myxinoids and Lampreys (Petromyzon) 
of to-day. 

Eleventh stage. — Selachii. Primitive fishes, closely 
allied to the sharks of to-day, with swim-bladder and 
two nasal cavities, two pairs of limbs (fins), and jaws. 

Twelfth stage. — Dipnoi or mud-fish. Vertebrata 
that are midway between Pisces and Amphibia, with 
gills and lungs, allied to the Ceratodus, Lepidosiren, 
and Protopterus of to-day. 

Thirteenth stage. — Sozobranchii. Amphibia with 
persistent gills, allied to the Proteus of the AcJeJs- 
berger grotto. 

Fourteenth stage. — Urodela or Sozura. Amphibia 
with transitory gills but persistent tail, allied to the 
Triton and Salamander. 

Fifteenth stage. — Protamnia. Forms intermediate 
between salamanders and lizards ; losing their gills 
completely and possessing an amnion. These repre- 
sent the parent form common to the three higher 
vertebrate classes, all of which have an amnion. 

Sixteenth stage. — Promammalia, the parent-forms of 
the class Mammalia. To these the Australian Mono- 
tremata (Ornithorhyncus and Echidna), with a cloaca 
and marsupial bones, but without a placenta, are most 
nearly allied. 

Seventeenth stage. — Marsupialia, allied to the kan- 
garoo and kangaroo-rat, with marsupial bones, without 
cloaca, without placenta. 

Eighteenth stage. — Prosimise, allied to the Loris 
(Stenops), and Makis (Lemur), without marsupial 
bones and cloaca, with placenta. 

Nineteenth stage. — Tailed apes or Menocerca. Catarr- 
hine apes with thirty-two teeth, with tails, allied to the 
Semnopithecus, Cercopithecus, and Colobus. 

Twentieth stage. — Men-apes or Anthropoida. Catarr- 
hine apes, without cheek-pouches or tail, allies of the 
orang, chimpanzee, gorilla. 

Twenty - first stage. — Ape-men or commencing 
men (Alali), allied to the lowest human races of the 



present time (Papuans, Hottentots, Australian negroes), 
but still destitute of true articulate speech. 

Twenty-second stage. — Men who have risen from 
the rudimentary condition of the preceding stage by 
the development of human speech and a simultaneous 
advance in brain-evolution connected therewith. 

When we have considered the stages as they are 
known to us, of the series of forms that the ancestors 
of man have assumed, from the Moneron to the Asci- 
dian, from the Amphioxus to the Gorilla, it behoves us 
to go a step further, and to discuss the different species 
of the human race and their unity of origin. As these 
questions have, during the last ten years, been very 
eagerly debated, a hurried glance may be of use at the 
light thrown upon them by the study of descent. But 
here it must be said that the decision in these matters 
is very indefinite and uncertain, inasmuch as the re- 
sults bearing on the questions obtained from compa- 
rative anatomy and ethnography, comparative philology 
and archaeology are contradictory and at cross purposes. 
Accordingly as the particular observer assigns greater 
weight to one or another ground of proof, his decision 
will vary. Here, more than at any other part, our 
present theory will seem still very unsatisfactory. 

Comparative philology, so important for the know- 
ledge of the true relationship of the younger branches 
of the human ancestral tree — e.g., the different branches 
of the Indo-Germanic stem — leaves us altogether in the 
lurch as far as concerns the momentous inquiry into 
the origin of the various human species. For it seems 
fairly certain, from many facts, that human speech was 
evolved after the marking-off of the different species 
had occurred. The primitive men whom we regard as 
the ancestral form common to the five to twelve species 
or races to be mentioned immediately, probably had 
no true speech. 

Next it may be remarked that the different forms of 
the human race that are usually regarded as races or 
varieties of a single human species (Homo sapiens), 
appear really to be good species. For the differences 
in color of skin, nature of the hair, and build 
of the skull, are not less marked than those differences 



by which many species of animal genera in a wild state 
are distinguished. 

It is well known that, according to Blumenbach, five 
races of man are distinguishable, which we may regard 
as so many species of the genus Homo. These are : 
(1) The white or Caucasian race (Homo albus) ; (2) 
The yellow or Mongolian (Homo luteus) ; (3) The red 
or American race (Homo rufus) ; (4) The brown or 
Malay (Homo fuscus) ; (5) The black or African (Homo 

The Englishman Pritchard, who after Blumenbach r 
has made the most extensive and complete inquiries 
into the so-called race-differences in man marks off 
three more races. He regards the Hottentots among 
the black African race, the Australians and Papuans 
among the brown Malays, as distinct races. This sepa- 
ration is justified not only by differences in color, in 
skin, in structure of hair, but by the different confor- 
mation of the skulls. 

The skull-conformation of man, on which of late 
extensive investigations and measurements have been 
made, presents, generally, three chief types, connected, 
however, by many intermediate forms : Ions: skulls, 
skulls of medium length, short skulls. The long skulls 
(Dolichocephali), whose most typical form is seen in 
the skull of the African negro, are elongated, and are 
laterally compressed. The short skulls (Brachyce- 
phali), most strongly developed in the Asiatic Mongols, 
on the other hand, appear shortened, cubical, com- 
pressed from front to back. Midway between the long 
and short skulls stand the Mesocephali, well shown in 
the American aborigines, and in many Europeans. 

The differences between dolichocephali and brachy- 
cephali, between woolly-haired and straight-haired 
peoples, between black and white skins that appear so 
marked in the extremes of the human race, are con- 
nected by a mass of gradations and transitional forms 
in such a way that it is quite impossible to separate the 
various races clearly one from another. But the same 
fact holds in regard to many animal forms that are gene- 
rally looked upon as " good species." On the one hand, 
therefore, we hold the races of man as "good species." 



But, on the other, we see in the intermediate tran- 
sitional forms sufficient ground for the conception of 
the unity of origin of all the species of man. By this 
it is not, however, implied that " all human beings 
have originated from a single pair." For in that long 
chain of many generations that formed the transition 
from the men-apes to the ape-men, and from these last 
to actual articulate speaking man, no single pair can 
be designated as u the first pair of human beings." 

The primitive human form, whence, as we think, all 
human species sprang, has perished this long time. 
But many facts point to the conclusion that it was 
hairy, dolichocephalic, dark, perhaps brown, of skin. 
Let us, for the time being, call this hypothetical species 
Homo primigenius. If, in addition to these, we regard 
the Eskimos as a peculiar species, we have in all ten 
different human species, four hairy, six smooth. As to 
the relationship by descent of these ten, the following 
approximate statements can be made. The first species, 
Homo primigenius, or the ape-man, the ancestor of all 
the others, probably arose in the tropical regions of the 
old world from anthropoid apes. Of these no fossil 
remains are as yet known to us, but they were probably 
akin to the gorilla and orang of the present day. The 
three following woolly-haired species, and of them the 
Papuan Negro mentioned next are, among living races, 
the nearest to Homo primigenius. Like these the 
primitive man was probably distinguished by crisp, 
woolly hair and dark or black skin. The skull was 
dolichocephalic and prognathous : the arms long and 
strong, the legs short and thin, the calf undeveloped. 
The hairy covering of the whole of the body was more 
strongly marked than in any of the living human 
species ; the gait only half erect, with bowed knees. 
It would seem that the region on the earth's surface 
where the evolution of these primitive men from the 
closely related catarrhine apes took place must be 
sought either in Southern Asia or Eastern Africa, or in 
Lemuria. Lemuria is an ancient continent now sunk 
beneath the waters of the Indian Ocean which, lying 
to the South of the Asia of to-day, stretched on the one 
hand eastwards to Upper India and Sunda Island, on 



the other westward as far as Madagascar and Africa. 
Of the various human species that evolved from the 
primitive men by Natural Selection in the struggle for 
existence, probably two, at first the most widely diver- 
gent from each other, conquered the rest. The one, 
woolly-haired, migrated in part westwards, to Africa, 
in part eastwards, to New Guinea. The other straight- 
haired, was evolved further to the North, in Asia, and 
also peopled Australia. Persistent forms of both stems 
are in all probability still surviving, of the former in 
the Papuans and Hottentots, of the latter in the Aus- 
tralians, and in one division of the Malays. 

We can next separate from Homo primigenius, as a 
second species, the Papuan men (Homo papua), and in 
this more extended sense, that we include under the 
name not only the more fully developed Papuan 
negroes of the present time, but also their lower, more 
ape-like ancestors that correspond with that woolly- 
haired division of the primitive species which moved 
from east to west. The present aborigines of New 
Guinea, New Britain, Salamon's Island, and so forth, 
and the vanished peoples of Tasmania (Van Dieman's 
land), seem to differ very slightly from these most 
ancient and least-exalted species of man. They all 
have woolly hair, and dark brown or quite black skins. 
They are all prognathous dolichocephali. Whilst the 
Papuans of to-day migrated from the original home of 
primitive man towards the rising sun, a branch of this 
stem probably moved westwards, and laid the founda- 
tions of the African population. The Hottentots seem 
to be direct descendants of this latter branch. 

The Hottentots (Homo hottentotus) we regard as a 
third species. Not only the Hottentots, but the Bush- 
men and certain allied and low races, all confined now 
to Southern Asia, belong to this division. Prichard 
has, ere now, separated these from the genuine negroes 
with whom Blumenbach had classed them. They are 
more nearly allied to the Papuans than to the negroes 
in many respects, especially in the tufted growth of 
the hair on the head. 

The true negro, or the man of Middle Africa (Homo 
afer), forms a fourth species, exhibiting the dolicho- 




cephalic skull in its extremest form. This species, 
like the three preceding, has crisp hair. The color is, 
for the most part, black, modified however, not infre- 
quently, to brownish and sometimes almost fair, yel- 
lowish brown as in the Hottentots. To this Ethiopian 
species belong the majority of the inhabitants of Africa, 
save the inland denizens of the north and the Hotten- 
tots of the south. Probably this species should be 
divided into two, the true negro (Homo niger), dweller 
in the mid-regions of Africa, between the equator and 
30° N. lat. and the Caffre (Homo cafer), dweller in the 
southern part of the continent, between 30° S. lat. 
and 5° N. lat. 

With the man of New Holland (Homo australis), a 
very low fifth species, we commence the series of 
straight-haired men. We consider the Australians of 
to-day as the lineal descendants almost unchanged of 
that second branch of the primitive human race men- 
tioned above, that spread northwards, at first chiefly in 
Asia, from the home of man's infancy, and seems to 
have been the parent of all the other straight-haired 
races of men. The Australians, like the four previous 
species, have dolichocephalic, prognathous skulls, and 
a black, blackish-brown, rarely clean brown skin. 
But they differ from these others in the smooth, straight 
hair, which is no longer woolly, as in the four previous 

As the Polynesian or Malay man (Homo polynesius), 
we may mark off in the sixth place next to the Austra- 
lian the species that still remains of Blumenbach's 
brown or Malay race, when the Australians and Pa- 
puans are eliminated. There is much likeness between 
these last and the Aborigines of Polynesia, that Austra- 
lian island-world that seems to have been once on a 
time a gigantic and continuous continent. To the 
Polynesian species belong the people of New Holland, 
Otaheite, and most of the small isles of the South Sea, 
as well as a great part of the Aborigines of Sunda 
island and Malacca. From this species also the Mada- 
gascar people sprang. They have, in general, a clear 
brown skin, like the preceding set, and a less pro- 
nounced dolichocephalism. Many of them are rather 


mesocephalic, many even brachycephalic. In these, 
and in other peculiarities, as e.g., the larger size of the 
brain, they seem to constitute the transition to the 
Mongolian and Caucasian races. 

The yellow or Mongolian man (Homo mongolus) 
forms a seventh species, covering the greater part of Asia. 
To it belong the Indo-Chinese, Coreo-Japanese, and men 
of the Ural and Altai, i.e., all the dwellers in northern 
and middle Asia, except the polar men ; also a great 
part of Southern Asia, and in Europe, the Lapps, Finns, 
and Hungarians. The broad shortened skull is very 
characteristic of this species ; certainly two divisions 
of it are mesocephalic, but none is truly dolichoce- 
phalic. The color of the skin is generally yellow or 
brownish-yellow, sometimes bright yellow, the hair 
straight, black, generally thin. In all probability the 
Mongolian species has evolved in Southern Asia from 
the Polynesian or Malay, and has spread thence east 
and north. 

We regard the Polar man (Homo arcticus) as an eighth 
species of man. Here we place the Eskimos and the 
Hyperboreans, the closely-related inhabitants of the 
North-pole regions in both the eastern and western 
hemispheres. Clearly this species has arisen, by special 
adaptation to the Polar climate, from some branch of 
another human race, that had wandered northwards 
and had spread east and west. Probably it was a branch 
of the Mongolian species that settled in this region, and 
became the ancestor of Polar men. Usually the Eski- 
mos are classed with the Mongolians, with whom they 
agree in the possession of a yellow complexion and 
straight, black hair. But they are separated from the 
Mongols by their long skull, and by other peculiarities. 

The red or American man (Homo americanus), the 
ninth species of the human race, includes the Abori- 
gines of the whole of America, always excepting the 
Eskimos in the northernmost part. These red-skins, 
as they have been called, have certainly not originated 
in America itself from an anthropoid ape of that con- 
tinent. They have assuredly wandered thither from 
the old world. The most probable descent of the 
American Aborigines is from Mongols that have passed 




over from Asia. The Mongolian, of all the other 
human races, stands nearest to the American. Most 
of the Aborigines of the new world are mesocephalic ; 
their skin is reddish or red-brown, more rarely yellow- 
brown. Some races in America seem to show that, 
besides the Mongols, Polynesians and mid-continental 
men wandered in the dim past into America and 
mingled with the former. 

We regard as a tenth and last species the so-called 
Caucasian or mid-continental race, the white man 
(Homo Mediterraneus). This species has evolved 
farther and to greater beauty than all others, mainly by 
adaptation to the favorable conditions of existence 
afforded by Europe with its temperate climate and ad- 
vantageous geographical conformation. In all proba- 
bility this human species evolved in Asia, and clearly 
in its south-western part, either from a branch of the 
Polynesian (Malay) species, or from some more ancient 
parent form. During the time that the mid-continental 
species was wandering from Asia to Europe, and later 
on also, after repeated immigrations, it divided into a 
great number of different branches and twigs, whose 
ancestral relationship is yet, for the most part, to be 
worked out by the investigations of comparative philo- 
logy. The Caucasians, pure and simple, the Basques, 
the Semitic peoples, and the Indo-European, can be 
marked off as four distinct races of this species. But, 
in addition to these, two " good species " of the human 
genus have to be separated and marked off from the 
mid-continental men. On the one hand is the Nubian 
(Homo Nuba) in Northern Africa (Dongolesi in the east, 
Eulati in the west). On the other hand are the Dravi- 
dians (Homo Dravida) in southern Asia, the aborigines 
of Ceylon and of the Deccan in Lower India. 

It is of no great moment whether the human race 
is divided into the ten species just enumerated, or into 
a larger or smaller number. In consequence of the 
variable nature and the brief duration of any organic 
species, this question will always be as difficult of de- 
cision in respect to the human genus as to the genera 
of plants and animals. This inquiry, further, has no 
bearing at all on the idea dealt with in this lecture, 



that of the single origin of the human race and the 
subsequent radiation of its different species from a 
single original place, a so-called centre of creation. Of 
the many important proofs that support this view, I 
refer only at the moment to the recent interesting con- 
clusions arrived at by Weisbach, as result of many 
comparative measurements of the body of different 
species of men (made by Scherzer and Schwarz in the 
Austrian Novara expedition). 

That immense superiority which the white race has 
won over other races in the struggle for existence is 
due to Natural Selection, the key to all advance in 
culture, to all so-called history, as it is the key to the 
origin of species in the kingdoms of the living. That 
superiority will, without doubt, become more and more 
marked in the future, so that still fewer races of man 
will be able, as time advances, to contend with the 
white in the struggle for existence. Of the ten species 
of man mentioned above, the first, primitive man, is 
dead this long time past. Of the nine others, the next 
four will pass in a shorter or longer time, the Papuan, 
Hottentot, Australian, American. Even now these four 
races are diminishing day by day. They are fading 
away ever more swiftly before the o'ermastering white 
invaders. On the other hand, the rest of the human 
races are still to the fore for the present. The Ethio- 
pian in mid-Africa, the Arctic man in the Polar 
regions, the Malay in southern Asia, the Mongol in 
Asia, will, for a long time yet contend, not without 
success, in the life-battle with the mid-continental 
man, because they can adapt themselves better than 
this last to the conditions of life in those regions, and 
especially to the climate. 

Melancholy as is the battle of the different races of 
man, much as we may sorrow at the fact that here also 
might rides at all points over right, a lofty consolation 
is still ours in the thought that, on the whole, it is the 
more perfect, the nobler man that triumphs over his 
fellows, and that the end of this terrific contest is in 
the vast perfecting and freedom of the human race, the 
free subordination of the individual to the lordship of 


Appendix I. 

Systematic Resume of the eight Vertebrate Classes. 

A crania 

Cranista . 

1. Lancelot or Amphioxus. 

2. Cyclostoma. 

f3. Pisces. 
Anamnia . . < 4. Dipnoi. 


(^Amniota ... -<7. Aves. 

(8. Mammalia. 

f Monorrhina 

Appendix II. 

The fourteen Mammalian Orders. 

I. Monotremata.. 
II. Marsupialia 

III. Placentalia... 

(1. Water-Monotremata 
(2. Land-Monotremata 

(3. Plant-eaters 

" (4. Animal-eaters 

Villiplacentalia ■ 

Zonoplacentalia ... 


( 8. Carnivora. 
\ !>. Pinnipedia. 

10. Edentata. 

11. Kodcntia. 

\ 12. Insectivora. 
I 13. Cheiroptera. 
[J 4. iSimiae. 

Appendix III. 

The Species of Anthropoid Apes and Men of to-day. 

1 f 

Ph j 

1 i 

Asiatic (Satyri) ... ( 1, 

Brachycephali ... ( 2. 

African Pongines ... ( 1. 

Dolichocephali ... ( 2. 


sotrichi). Mostly 
Brachycephali or^ 
Mesocephali, afew 
Dolichocephali ... 

Lesser Orang (Satyrus morio). 
Greater Orang (Satyrus orang). 
Chimpanzee (Pongo troglodytes). 
Gorilla (Pongo gorilla). 

Papuan (Homo papua). 
Hottentot (Homo hottentotus). 
Caffre (Homo cafer). 
Negro (Homo niger). 
Australian (Homo australis). 
Malay (Homo polynesius). 
Mongol (Homo mongolus). 
Polar man (Homo arcticus). 
American (Homo americanus). 
Caucasian (Homo mediterraneous). 



" In the study of Nature we must ever treat the one as we treat 
the many. There is no within or without : that which was internal is 
external. With speed then seize this holy, open secret." 

TO choose division of labor as the subject of a sci- 
entific lecture may seem strange, or at least unne- 
cessary, to not a few. For nearly every one is supposed 
to be sufficiently well versed in the nature and working 
of this important principle from the experience of 
every-day life. It is only necessary to cast a glance 
over any association of human beings in our civilised 
age to see everywhere division of labor, the various 
activities of the individuals that are working together V 
for a common end, as one of the mightiest forces in 

In every workshop, in every factory, on every farm, 
a wise apportioning of different tasks to different 
workers is the first essential for a prosperous issue to 
the labor. Division of labor is of such fundamental 
importance in the evolution of the civilised life of 
man that its extent may be used as measure of the 
stage to which civilisation has advanced. The savage 
peoples, who have remained even down to the present 
day, standing, as it were, at the foot of the ladder, are ^ 
as wanting in division of labor as in culture. Or that 
division is, as in the majority of animals, limited to 
the diverse occupations of the two sexes. On the other 
hand, we can find the main cause of the gigantic ad- 
vances made during the last fifty years in our civilised 
life directly in the extraordinarily high degree of divi- 
sion in our modern work, especially in the regions of 
natural science and its practical application. Modern 



science, with its microscopes and instruments, modern 
commerce, with its railroads and telegraphs, modern 
war, with its needle-guns and mines — these are possible 
only as results of the infinite, wide-reaching division 
of labor of this our day, are possible only in that each 
instrument, each machine, each weapon, sets in motion 
in various wise, hundreds of human hands. How 
many new forms of labor and trades have evolved of 
late, what transformations have these wrought in the 
products of modern toil and in the characters of the 
toilers ! 

Besides these relations of division of labor that are 
known to the many, in Nature and in the life of man, 
occurs a series of special instances of that division not 
less significant but none the less wholly ignored, as a 
rule. Nay, strange as it may seem, these phenomena 
of division of labor that are the most important of all, 
the most widely-reaching, are actually not known to 
the majority of men. They have been in part dis- 
covered during the last ten years by the labors of 
scientific men. To this category belong especially those 
cases called by naturalists differentiation, specialisa- 
tion, polymorphism of individuals, divergence of 
characters. 1 

I want in this lecture to awaken an interest wider 
and well deserved in these cases, of which so little is 
known, and the knowledge of which is of the highest 
moment to the understanding of human life. 

Here it seems wisest to start from those conditions 
encountered in the life of animals lower than man, 
that appear most nearly related to the division of labor 
as know T n in human life. For here, as in so many other 

1 Darwin, in the fourth chapter of his celebrated work on the 
*• Origin of Species,"' gives the name of "divergence of character" 
to that kind of division of labor that occurs between contemporaneous 
individuals of the same species in the same place, and which in their 
struggle for existence leads to the formation of varieties, and further 
of new species. This divergence of character, like the differentiation 
of organs, the key to comparative anatomy, depends as a morpholo- 
gical process on physiological division of labor. In both cases, as I 
have clearly shown, in the nineteenth chapter of my " Generelle 
Morphologic *' (Berlin : Keimer, 18G0, vol. ii., p. 253), the essence of 
the process is "the production of unlike forms from like foundations/' 



cases, the unprejudiced eye of the naturalist sees that 
human life-conditions are repeated in the life of lower 
forms of animals, and that the simpler structure of 
these last leads to the right understanding of the more 
highly-developed organs of man. \ Unfortunately it is 
too clear that the prejudice is still* wide-spread that 
sees in the phenomena of human life something alto- 
gether special and supernatural, and that blinds men 
to every relationship and comparison between the 
phenomena of man and of other animals. Neverthe- 
less the knowledge of the common basis of'all phaeno- 
mena, including those that have to do with man, a 
knowledge advancing with gigantic strides, is day by 
day destroying that artificial barrier. I£.ji.s_ teaching 
the unbiassed inquirer that man is certainly the orga- 
nism foremost in advancement, highest in development, 
but still only an organism having form and composi- 
tion, functions and origin, in common with other 
animal organisms. The same laws of Nature, eternal, 
immutable, tTiaE reign over plants and animals, govern 
the whole life of man in its advancing Evolution. 

"We must all trace out the circle of our destiny 
according to mighty, eternal venerable laws." 

The phenomena of the division of labor are espe- 
cially fitted to strengthen this conclusion. For higher 
organisation in animals, as in man, is essentially 
dependent upon the more marked degree of division 
of labor. Very many kinds of animals exist in whom 
the division of labor amongst the gregarious individuals 
is, as among the rudest savages, confined to that most 
simple social form, the diverse occupations and 
specialisations of the two sexes — in a word, to mar- 
riage. 1 But there are also many kinds of animals in 

1 Marriage — the different functions and specialisation of the two 
sexes — on which the family of man and other animals depends, is one 
of the earliest and most widely-spread forms of social division of 
work. In most animals this has led, as it has in man, the highest 
animal, to the remarkable differences in the bodily form and mental 
character of the two sexes. But these differences fail us in many of 
the lower animals, as the two sexes — recognised by the different forms 
of the reproductive organs — are not distinguishable externally. On 
the other hand, the sexual division of labor that constitutes the original 



whom the division of labor amongst the individuals 
living together in a community goes much further, and 
leads to the organisation of those higher associations 
that we call states. 1 

The best known of these animal communities is the 
\ monarchical bee-state. At its head is a queen, in the 
strictest sense of the word, the mother of all her people- 
There are from 15,000 to 20,000 workers, from 600 to 800 
drones or males. All the burden and toil of the hive 
falls upon the busy workers : the gathering of pollen r 
the preparation of honey and wax, the making of the 
comb, the care of the young. The lazy drones form 
the courtly household of the queen, and, like such a 
household among men, live only for pleasure, and have 
as sole office the reproduction of the species. 

The economy and the remarkable social relations of 
the bee state are so generally known that we need not 
here lose time by a further consideration of them. 
Less well known, but still more interesting, are the 
communities met with in many other species of insects, 
especially the ants and the Termites, or white ants.. 
In these insects we find in one and the same corn- 

essence of marriage has gone much farther in many animals than in 
man, and has given rise to bodily structures so wholly different in the 
two sexes that zoologists, before they knew their connexion, have very 
often described the male and female of a species as two distinct 
species, or even as animals of two distinct classes — as, e.g., in many 
of the lower parasitic Crustacea and other parasitic animals. The 
moral basis, by which, in the more highly-civilised men, marriage has 
been improved to a great extent, is altogether wanting in many of the 
lower savages — the American Indians, many Negroes, the Australians, 
etc. Among these brutish men, with whom the female holds the 
position and receives the treatment of a useful domestic animal, there 
can be no talk of marriage on any moral basis. This could more easily 
be discussed in regard to the lower animals that live in rigid mono- 
gamy, as the doves, parrots and many other birds. Sexual choice, the 
sexual selection of ])arwin, has, in addition to the sexual division of 
labor, influenced and altered in notable fashion the two sexes. On 
this the nineteenth chapter of my " Generelle Morphologic " treats 
(vol. ii., p. 244). 

1 On the communities of the lower animals — e.g,, of the bees and 
the ants — and their analogies witli human states, cf. especially the 
" Untersuchungen der Thierstaaten " of Carl Vogt and Ludwig 
Buchner's " Geistesleben der Thiere " [translated by Annie Besant as 
" Mind in Animals," vol. i. of this series]. 



munity at least three, often four, and even five, differ- 
ent forms of individuals that have been evolved as 
results of a regular division of labor. The three forms 
always present in an ant community are : 1. the winged 
males ; 2. the winged females ; 3. the wingless 
workers. Of these the last far exceed the other tw^o 
in number. If four forms are found the wingless 
workers divide again into two groups, workers pure 
and simple and soldiers. Each of these is of very 
different build from the other. 

As with the bees, all the burden and toil of life falls 
on the indefatigable workers among the ants and Ter- 
mites. The three other classes live for the most part 
for pleasure. The winged males and females that have 
as sole function the reproduction of the species, amuse 
themselves in the fine weather with pleasure flights 
and dances in the sunny air. The soldiers, whose func- 
tion is to guard the state, can take no part in pleasures 
such as these, for they are, like the workers, wingless. 
So much the more, then, do they give themselves 
up to eating the sweet food, with which the commu- 
nity is continually supplied, even to excess, by the 

The food of ants consists, as is well known, of all 
kinds of plant and animal matters. Sweet juices are 
the favorite food, and amongst these stands foremost 
as the choice national dish a honey-like juice that the 
Aphides or plant- lice furnish. These little insects have 
two ducts opening on the back, whence this pet 
delicacy of the ant exudes. The ants suck the sw T eet 
Aphis honey just as we take milk from the cows. By 
stroking the Aphides with their feelers the ants induce 
them to yield up their honey. The most energetic 
farmer can be no more careful in the rearing and 
breeding of his cows than the ants in the rearing and 
breeding of their milk-providers. If a branch of the 
shrub on which the plant-lice dwell becomes withered 
the ants carefully remove the Aphides on it to a fresh 
green branch. They construct very cleverly covered 
ways from their nest to the shrub. Nay, they even 
place such Aphides as are housing at the base of the 
nest side by side with those already in their nests, 



building there special stalls, so as to have the valuable 
milk-producer always at hand. 

Whilst thus some of the workers in the ant community 
carry on the breeding of animals or provide the hive 
with other food, another part is concerned in the main- 
tenance, cleansing, and extension of the dwelling 
wherein all the people of the ant community are 
gathered together. What are our mighty palaces, 
castles, cathedrals, hotels in comparison with these 
buildings, in which many thousands of individuals 
live together in peace ? It is true that on the exterior 
the houses of most species of ants seem sufficiently rude 
and shapeless. But within them lies hidden a labyrinth 
of many hundred winding ways, corridors, roofs, con- 
nected in most convenient fashion by thousands of 
rooms and chambers. Many of these are nurseries in 
which the young progeny are reared. 

The care of these latter, and in especial of the chry- 
sales, generally known by the false name of ant-eggs, 
falls to another part of the workers. These nursemaids, 
filled with the tenderest love for their charges, drag 
them out into the fresh air in fine, sunny weather ; but 
as soon as the chill of evening comes on, they take 
them back again into the warm interior of the hive. 
The soldiers, though they are bigger and stronger, take 
no part in all these arduous labors. 1 

There are, besides, other species of ants, amongst 

1 Division of labor reaches its highest development in the Sahubae, 
the leaf-carrying ants of the Brazilian woods ( (Ecodoma cppha/otes). 
In these there are not less than three kinds of workers, altogether 
distinct in size and shape ; so that, including the winged males and 
females, not less than five different forms of ants are living together 
in the same community. The larger number are small -headed 
workers, that strip the trees of their foliage, biting off the leaves and 
carrying them off to line their artificially constructed homes. Amongst 
these, larger workers are running about with very large, smooth, 
bright heads, that seem to superintend and direct the work, and 
perhaps also to serve as protection for the little workers. Nothing- 
certain is known up to the present time as to the significance of the 
third form of workers, that differ from the second in the thick, hairy 
covering of the colossal head, and one large median frontal eye. On 
these Sahubaa, and on the robber-ants or Ecitones, of. the very inte- 
resting observations of Walter Bates, in his excellent work of travel. 
" The Naturalist on the Amazons. ' 


whom all the workers have become soldiers. These, 
therefore, have already realised that human ideal of 
civilisation of this later age, the modern military state. 
Hence these soldier-communities are obliged either to 
carry off the domestic workers as slaves, or to live 
by theft and plundering. The notorious South-Ameri- 
can robber ants of the genus Eciton follow the latter 
plan. Here, again, we encounter in this species four 
different forms, the winged males and females, and two 
kinds of wingless workers, differing widely in shape 
and size. The smaller workers, who form the chief 
part of the whole Eciton community, serve in a body 
as common soldiers. The larger workers, on the other 
hand, distinguished chiefly by a very large head and 
by immense mandibles, serve as officers to the army. 
Generally there is one officer to a company of some 
thirty men. On the march, the officers are ranged on 
both sides of the long column of warriors, and often 
climb to high standpoints, whence they can overlook and 
direct the march of the troops. Commands and plans, 
as is general with all intellectual communications, are 
given in these, as in other ants, so far as we know, not 
by voice, but by a speech of gesture and touch. Espe- 
cially are the antennae of use, partly as telegraphic 
instruments, that give by their vibratory movements 
signals at a distance, partly for the communication, by 
immediate contact, of wishes, feeling, and thanks to 
those at hand. 

The army of robber ants on its march, like the Van- 
dals and Huns in the age of migrating peoples, devas- 
tates all regions that it traverses. They are with 
reason objects of exceeding dread to the Brazilian 
Indians. All living things that cross their path are, 
without remorse, without mercy, seized and slain. 
Spiders and insects of every order, especially larva? 
and pupae, even birds and small mammals succumb to 
their attack. The man who is unfortunate enough to 
happen upon such an army of nomads is surrounded 
in a moment by dense black swarms, that run by the 
thousand, with incredible anger and swiftness, up his 
legs, and bite into his flesh with their sharp teeth. The 
one remedy is to run with all swiftness to the rear of 


the army, and to tear away at least the distal part of the 
warrior that has bitten his way into you. Head and jaws, 
as a rule, remain embedded in the wound, and often 
give rise to unpleasant sores. 

Terrific, blood-thirsty, as are these nomad hordes 
on the war path, they are equally amusing and merry 
when they bivouac ; when satiated with food, in the 
best of humors, they give themselves up to peace and 
pleasure in the sun-lit sylvan places. At first they 
clean their antennae with their front legs. They lick 
the dust off one another's hind legs. In this they give 
evidence of petulancy and humor, and not unfre- 
quently the soldiers, carrying their sport beyond 
bounds, fall a-brawling. 

More noteworthy even than the military commu- 
nities of the Brazilian Eciton are the slave commu- 
nities, the so-called Amazon states, met with among 
several of our native ants, especially the blood-red and 
light-colored ants (Formica rufa and Formica ru- 
fescens). Among these ants we find only three forms, 
i.e. only one form of wingless workers besides the 
winged males and females. But these not only labor ; 
they steal, from the hives of other smaller blacker ants, 
the pupae. These they rear ; these they compel to 
perform, like slaves, all the work of the strange hive 
in which they are. As a rule the slave foray of these 
Amazon ants is carried on in such a way that the 
whole force of the black ants is engaged in free fight 
in the open by the main body of the lighter-colored 
ants, and in the meantime a small band of the latter 
falls upon the home of their foes, and steals the pupae 
from the hive deserted of its defenders. The study of 
the bitter contest is very interesting. The wounded, 
even the corpses of the warriors that have fallen, are 
as once on a time in the Trojan War, carried off by 
their friends from the bloody turmoil of battle and 
brought into safety behind the lines of the contending 
hosts. But the most remarkable thing is that the black 
slaves that are bred from the stolen pupae not only do 
all the work of the nest for their masters, building, 
food-gathering, attending to and rearing the young, but 
even later on aid them in their marauding excursions, 



and carry off the stolen youth of their own race into 
slavery. 1 

We find, then, in the Amazon communities of the 
German ants the same system of slavery that was 
brought to an end in the States of North America by 
the late war. Generally laborious efforts are made to 
designate as the outcome of " instinct " these and 
similar arrangements in the lower animals that astound 
man by their undeniable identity with his own insti- 
tutions. There is a belief that the word " instinct " is 
an explanation. Few words have led to so vague, so 
perverted a conception of a vast domain of momentous 
phenomena as this word " instinct." When the word 
is used, the notion is that every kind of animal was by 
a single act of creation sent into the world with a 
definite sum of impulses and faculties, that it had 
a special mode of existence marked out for it by its 
creator, a sort of inexorable law of life, and that it 
must live after this definite plan, rigidly, unalterably. 
Nothing is more erroneous, more opposed to the truth 
than this wide-spread idea. Just as the individual 
species of animals have not been created as they now 
are, so their special instincts, the mental stock in trade 
of the species, have not been thus created. They have 
evolved from one common fundamental condition 
by division of labor in the central nervous system of 
the different species of animals, as their whole organi- 
sation has evolved. 2 A distinguished philosopher has 
well said that he who draws a line between instinct 
and reason gives in himself, in doing thus, the best 

1 The slave states of the Amazon ants, beyond dispute the most 
remarkable social arrangement in the wonderful ant- world, had been 
observed during the last century by the celebrated Huber of Geneva. 
Later these observations were confirmed by Latreille, Carl Vogt, and 
several other zoologists. See Carl Vogt's " Vorlesungen liber niitz- 
liche und sch'adliche, verkannte und verlaiimdete Thiere " and Ludwig 
Buchner's " Geistesleben der Thiere." 

2 The idea of creation is wholly unscientific. In its place true 
science puts the idea of Evolution. On this point see the first lecture 
(p. 6) of my " Nattirliche Schopfungs-geschichte," a series of popular 
scientific lectures on the study of Evolution in general, on the writings 
of Darwin, Goethe, and Lamarck in particular, as to the application 
of Evolution to the origin of man. and on other related fundamental 
questions of science. (Berlin : Eeimer, 1868. Sixth Edition, 1875.) 




proof that he has never observed carefully with keen, 
unbiassed eye the life and labors of the lower animals, 
and especially of insects. 

If the state organisation amongst ants and bees just 
described, if, as a whole, all the different economical 
and life relations of the lower animals, if in especial 
their division of labor are to be regarded as the outcome 
of "blind instinct," with equal justice must we call it 
" blind instinct," when the Eskimos make their tents 
of reindeer skins, the North-i^merican Indians theirs 
of buffalo-hides, the red-skins of Brazil theirs of palm- 
branches and the leaves of the banana. We must, in 
like fashion, call it " blind instinct " that many South- 
Sea Islanders live almost wholly on fish, that the 
Chinese eat little but rice, and the Gauchos of the South 
American pampas little but meat. We must, in like 
fashion, call it "blind instinct," that the peoples of 
Europe, with a single exception, have retained the 
monarchical form of government, like the bees ; and 
that the peoples of America, again with a single excep- 
tion, have preferred the republican form of govern- 
ment, like the ants. 

The fact is that in this, as in all things, custom and 
adaptation to surrounding conditions determine the 
mode of life and social arrangements of man as of 
other animals, and that this mode of life becomes at 
last, by usage and habit, second nature. It takes root 
in this way the more firmly in the race the larger the 
number of the generations through which it has been 
transmitted. Adaptation and heredity, in their eternal 
mutual action and re-action, i.e., Natural Selection in 
the struggle for existence, are the eternal formative 
impulses, the Evolution forces, that give rise, by purely 
mechanical laws, to all the endless variety, both in the 
organisation and modes of life of animals, and in their 
soul-life, the so-called instinct. 1 

Every scientific man who is intimately acquainted 

1 I have shown in the eleventh lecture (p. 203) of my "Naturliche 
Schopfungs-geschichte," that the action and re-action between the in- 
ternal formative impulse of heredity and the external one of adap- 
tation, can give rise to all the endless variations of the plant and 
animal organisation in a purelv mechanical way, i.e., according to 



with the history of Evolution in animals is convinced 
that all these various ant species, with their varied 
kinds of labor, have evolved from common ancestors, 
long dead, who had not these different divisions of 
work. Those red primordial ants that lived for many 
thousands of years, probably in the Cretaceous epoch, 
had as slight a foreshadowing of the advanced division 
of labor, seen in the various ants of these days, as our 
old German forefathers of the Stone Age had of the 
lofty civilisation of the nineteenth century. The men, 
like the ants, have slowly, gradually toiled up the pain- 
haunted path of advancing Evolution. Even at this 
hour some ants exist that know nothing of the highly- 
developed division of labor met with in the civilised 
ant communities, and bear to these last a relationship 
none other than that of the red aborigines of Australia 
and Africa, to the civilised nations of our own time. 

If we turn to cast a glance over the mental Evolu- 
tion of man in that grey past in which the ancestors of 
the civilised races of to-day had not advanced beyond 
the brute conformation of the rudest savages, the Aus- 
tralian negro, the Papuan, the Bushman, etc. ; if we 
observe how slowly, how gradually the human race 
has, in the struggle for existence, gained by infinite 
effort its specially human character, we see clearly 
that the soul-life of man has evolved from the same 
crude foundations as that of other animals, and that ] 
the " instinct " of these latter is distinct from the 
reason of man in quantity only, not in quality, in de- 
tails, not in essence. This holds as to the soul-move- 
ments of sensation and will as of those of thought, of 
judgment, of reason. But it will be yet clearer to any 
one that the phenomena of division of labor referred 
to have evolved in human life in the same way as they 
have in that of lower animals, in consequence of kin- 
dred conditions of adaptation, if he studies the phe- 
nomena of comparative division of labor that are even 
now to be discussed. 

Let us pass in thought from the hot tropical woods 

physical and chemical laws. In my " Allgemeine Entwickelungs- 
geschichte (vol. ii., of " Generelle Morphologie," p. 223, et. seq.), I hare 
given proofs of this in detail. 




Fig. i. 

A free -swimming Medusa (Carmarina hastata) of the family a. Nerve-ring, within the margin of the um- 
brella ; a'. Radiating muscles ; b. Auditory vesicle ; c. Cir- 
cular vessel within the margin of the umbrella ; e. Centri- 
petal canals given off from the last ; g". Triangular, leaf- like 
ovary ; h. Circumferential organs of the umbrella ; h. Sto- 
mach ; I. Gelatinous mass of the umbrella ; o. Mouth ; p. 
Peduncle of the stomach ; t. Tentacles or feelers ; u. Outer 
wall of the umbrella ; v. Velum or swimming membrane. 



of Brazil, where the robber ants and the Sahubae lead 
their checkered lives, to the cool shore of our northern 
German coasts, when a fresh north wind has thrown 
up on the sandy strand a number of so-called sea- 
nettles or jelly-fish (the Medusas of the zoologists). 
Whoever has walked with eyes open on the shore of the 
Baltic or of the North Sea, is acquainted assuredly with 
these jelly-like animals that are often cast up by the 
thousand from the waves. When we see them lying 
there in heaps, a slimy, shapeless mass of jelly, we can 
form no clear idea as to the wondrous beauty that these 
Medusas swimming in the sea, exhibit. But if they are 
placed with some of the water in which they swim in 
a large glass vessel, we are filled with wonder at the 
grace of their swift movements, the delicacy of their 
shimmering hues, the elegance of their flower-like form 
(Fig- 1). 

The commonest of our large North-German Medusae 
is called Aurelia aurita. Fig. 2 in vertical section, 
Fig. 3 seen from below. 

An eared jelly-fish (Aurelia aurita), from the Baltic, in ver- 
tical section, a. Gelatinous umbrella ; o. Mouth ; o". Two 
of the four oral tentacles, beset with sexual buds ; o'. Section 
through their bases (the pillars of the mouth) ; g. Ovary ; 

Stomach ; h". Branching vessels running from the sto- 
mach to the margin of the umbrella ; this last beset with 
many delicate tentacles. 

The gelatinous, transparent body of this Aurelia has 
on the whole the shape of a shallow glass bell. In the 
middle of its under surface is the mouth (Fig. 2 o), sur- 
rounded by four long, very mobile tentacles (Fig. 2 o", 



Fig. 3 b.) From the circumference of the bell-shaped 
disk or umbrella hang many very fine tentacles (Fig. 
3 t) Further, eight sensory vesicles (Fig. 3 a) are 
placed at regular intervals within the margin, and 
probably function as both eyes and ears. The mouth 
o, leads into a stomach (Fig. 2 k, Fig. 3 v), whence 
many radiating, branching, digestive canals (Fig. 2 k, . 
Fig. 3 gv) run to the margin of the disc, uniting there 
into a single circular vessel. Around the stomach lie 
four pouches (c) placed in the form of a cross. In these 
the eggs of the Medusae are formed (Fig. 2 g, Fig. 3 n.) 

Fig. 3. 

The same, seen from below : the lower half has been removed. 
a. Sense-vesicles (eyes and ears) on the margin ; t. Tenta- 
cles ; b. Oral tentacles ; v. Cavity of the stomach ; ov. Ovary 
in the lower wall of the last ; gv. Branching radiating 
canals, running from stomach to margin and there combin- 
ing in a circular vessel. 

The division of animals to which the Aurelia and 
its allies, the jelly-fish, belong, bears the name of Hy- 
dromedusae. To the same group belong the Hydroid- 



polyps, altogether dissimilar from the free-swimming 
jelly-fish in external aspect, attached to the sea-bottom, 
or sedentary on seaweed. A solitary little animal, be- 
longing to this group, the small fresh- water polyp or 
hydra (Fig. 4), lives, and is widely distributed, in our 
ponds and ditches. This elegant little creature is found 
very frequently on the under aspect of duckweed or 
the leaves of the water-lily. In the contracted state 

Fig. 4. 

(Fig. 4 to the left) it is a green or orange-red little 
mass, the size of a pin's head. Extended (Fig. 4 to the 
right) it is a thin thread, an inch in length. By one 
end it is firmly fixed. At the other is the mouth, sur- 
rounded by a circle of from four to eight tentacles. 
The mouth in this case leads into a simple gastric 
cavity. Our fresh-water polyp reproduces itself in 



the simplest fashion. Either by eggs or by budding, it 
is ever producing creatures like itself. But in the sea 
live many hydroid polyps, scarcely distinguishable 
from the fresh-water one, but nevertheless reproducing 
themselves in a manner very remarkable and very 
different, i.e. in connexion with the Medusae just 

For from the eggs of the Medusae other Medusae do 
not arise, but hydraf orm polyps, and these latter pro- 
duce by gemmation not polyps but Medusae again. 
Hence in these Hydromedusae the daughter does not 
resemble the mother, but the grandmother. The first 
generation is like the third and the fifth, the second 
generation as the fourth and sixth. But the two kinds 
of beings of one and the same species are so different 
that they were regarded in earlier days, ere their rela- 
tionships were understood, as two entirely different 
groups of animals, Medusae and Polyps. 

Among the higher Medusae or Acraspeda, to whicn 
our Aurelia (Figs. 2, 3) belongs, a hydra-polyp is de- 
veloped from the egg (Fig. 5) and multiplies by gem- 

Fig. 5. 

Hydra-polyps (Seyphistoma), developed from the eggs of an 
acraspedote Medusa, a. A polyp with three buds. b. One 
tentacle of the last, strongly magnified, c. A polyp with 
stolons, whence two buds take origin. 

From the oral extremity of each bud grows a series 
of young Medusae, forming a body not unlike the cone 
of a fir (Strobila, Fig. 6.) 

One after another the ripe young Medusae detach 
themselves from the inferior part of the Strobila and 
set out on their wanderings as the far more highly 
organised acraspedote form (Figs. 2, 3.) 



Fig. 6. 

Two Medusae cones (Strobilae), developed by distal gemmation 
from two hydra polyps (Fig. 5 c.) Each Strobila consists of 
eight young Medusae, placed in a row, one after the other, 
like links in a chain, a the older, b the younger Strobila. 

Amongst the lower or craspedote Medusae, on the 
other hand, to which the Carmarina belongs, the polyps, 
or first generation that are developed from the Medusae 
eggs, give rise, by gemmation, to a tree-like, branching 
or creeping colony. From this colony, again, Medusae 
spring, detaching themselves later on ; or special buds 
(metamorphosed polyps), in each of which many 
Medusae originate by gemmation (Fig. 7 /.) 

Such a colony may, therefore, bear two different or- 
ganisms, wholly unlike externally, that have sprung by 
division of labor from the same original hydraform 
polyp. The one set of polyps, the long-stalked food 
polyps (Fig. 7 a, b) are concerned only with eating, 
drinking, digestion, but can form no more eggs. They 
possess an open mouth and a circle of tentacles, but 

Fi 7. 

A creeping polyp colony (Campanularia Johnstoni). On the 
creeping root stock (e) are placed two hydra polyps that are 
as result of division of labor wholly diverse in their develop- 
ment. The long-stalked are food polyps (a~d), the short- 



stalked are reproductive (/.) The latter form buds that de- 
velop into Medusae, and swim away (g.) The former are 
able to withdraw (b) their outstretched bodies within a 
horny envelope (c.) Their stalk (d) is annulate at top and 

they have lost the power of reproduction. The second 
set of polyps, the short-stalked reproductive ones, or 
" nurses " (Fig. 7 /), have lost their tentacles and their 
oral openings are closed. On the other hand, many 
buds arise from the wall of their gastric cavity, that 
detach themselves later on (Fig. 7 g), and develop into 
free swimming Medusae that still later carry eggs. 

Fig. 8. 

A Medusa (Eucope). In the middle of the bell-shaped body 
hangs the stomach, whence four digestive canals run to the 
margin of the umbrella. In the middle of these canals 
lie the eggs (g.) From the margin of the umbrella (&) 
hang four tentacles. Behind these are eight auditory 
vesicles (a.) 



A similar series of alternations between two or even 
three generations widely differing is very general 
among the lower animals, and is known as alternation 
of generations. But these remarkable alternations may 
be regarded as the result of division of labor, in short, 
as a division of labor that comes into play in the domain 
of developmental life. 1 The two distinct forms, the 
Medusae, from whose eggs the polyps arise, and the 
polyps, from whose buds the Medusae arise, are two 
different forms of one and the same species, evolved 
through division of labor from a common ancestor, in 
the same fashion as are the different kinds of workers 
in the ant community. 

The clearest light is thrown upon the successive 
alternations of generations in the Medusae and polyps 
by the marvellous free-swimming colonies of the Hy- 
dromedusae, that the zoologists call Siphonophora or 
jelly-fish communities. These are amongst the most 
magnificent sights of the southern seas. They appear 
at certain times in dense swarms in the Mediterranean, 
e.g. in the straits of Messina. As a whole they may be 
compared to a swimming flower-stem laden with beau- 
tiful flowers and fruits of every hue, all whose parts 
seem fashioned out of transparent crystal, whilst they 
have an animal's life, soul, spontaneous movement, sen- 
sation, consciousness. Let us look a little more closely 
at the complex organisation of one of these wondrous 
colonies. (See the succeeding figures from 9 onwards, 
and the accompanying explanations. 2 ) 

1 Rudolph Leuckart, in his work on " Polymorphismus der Indi- 
viduen, oder die Erscheinungen der Arbeitstheilung in der Natur " 
(Giessen: Ricker, 1851), has clearly explained this idea, that "the 
alternation of generations in animals has originated through division 
of labor in the domain of developmental life." Correct as this idea is 
in many cases, it cannot lay claim to accuracy in all instances. There 
are, in fact, many cases of alternation of generations that are clearly 
to be looked upon as periodic reversions or atavism, and are explicable 
on the principle of suspended or latent heredity ("Generelle Mor- 
phologic," vol. ii. p. 181, and " Natiirliche Schopfungsgeschichte," 
p. 161.) 

2 A more detailed account of the free- swimming Siphonophora 
colonies and their marvellous division of labor may be found in the 



work of Leuckart, already referred to, on Polymorphism (p. 119), and 
at p. 104 of Carl Vogt's essay on^the complex animal colonies (third 
part, p. 162.) 



lyces or swimming balls, o. Opening of the umbrella. 
t. Tactile polyps, g. Egg-forming female individuals, n. 
Nutritive individuals. 

Around one of the elastic central stems, often many 
feet in length, of the common axis are arranged hun- 
dreds, often thousands, of Medusae and polyps, that 
have by division of labor attained very different shapes 
and structures. The central stem itself is no other than 
a very elongated simple polyp body, closed below, but 
expanded above into a swimming bladder, or pneuma- 

Fig. 10. 

Swimming bladder or air bladder of a Rhizophysa. c. Ex- 
ternal wall. /. Large air bladder enclosed in an air sac. / 2 . 
Tufted appendages from its inferior aspect, xy. The two 
body layers, formed from the primary layers of the blasto- 
derm, x. Endoderm or digestive layer, y. Ectoderm, or epi- 
dermal layer. 



tophore, filled with air. This, buoyant upon the sur- 
face of the sea, supports the whole colony (Fig. 10.) 
Below this air vesicle is a double row of bell-shaped 
Medusae. These, by their common contractions, that 
are subordinated to the will of the animals, move the 
whole community through the sea, and hence take the 
name of motor polyps (Fig. 9 m.) Each motor polyp 
(Figs. 11, 12) is essentially a simple Medusa, but with- 
out arms, without organs of digestion or of reproduc- 
tion. Whilst they are formed solely for swimming, 

Figs. 11 and 12. 
A swim-bell of a compound Hydrozoon (Forskalia.) Fig. 11 
from below, Fig. 12 side view. a. Opening of the cavity 
whence in swimming the water is driven out. b. Swim- 
membrane or velum, c. Circular vessel in the margin of the 
umbrella, d. Four radial vessels, e. Swim-sac. /. Cartila- 
ginous tissue of the bell. g. Eye-spot. 

they have lost the other functions of the} Medusae. 
Their motion forwards results from the repulsion of 
the sea- water, which is expelled by regular contractions 
from the opening of the bell (Fig. 11 a, 12>) as they 
swim along. 



Below the double row of swim-bells are a number of 
different animals of various colors ornamenting the 
whole of the inferior part of the stem. First there is a 
dense mass of leaf-like or bract-like pieces, grouped 
round the axis, like the leaves of a fir-cone. Beneath 
these, when danger threatens, the other individuals can 
flee for protection. These so-called bracts or hydro- 
phyllia are retrograde Medusae that have taken on the 
sole function of passive organs of protection, of shield- 
bearers (Fig. 13 They consist for the most part 

Fig. 13. 

A bract or shield-bearer (h) of a hydra-colony (Stephanoinia.) 
A nutritive polyp (b) and several tactile polyps (h) are pro- 
tected by it. 

simply of a cartilaginous jelly-like mass traversed by a 
nutritive canal. Beneath their umbrellas we find hid- 
den a number of pear-shaped bodies, with a mouth 
opening, that seizes food greedily, and with digestive 
cells of a hepatic nature. It can attach itself firmly by 
suction by means of its octagonal oral margin (Fig. 
14 /), which is capable of extraordinary expansion. 
These have, as feeding polyps, the duty of taking in 


Fig. 14. 

A nutritive polyp and tentacles of a hydra colony (An the - 
modes.) a. Point of attachment of the polyp to the stem ; 
h. Body-wall of the polyp ; c. Its gastric cavity ; d. Hepatic 
cells, e. Proboscis. /. Month aperture extended into the 
form of an octagonal disk, and attached by suction, g. Wall 
of the tentacle, h. Its cavity, i. Secondary tentacles. 1c. 
Bell-shaped investment of the thread-cell battery. I. m. 
Terminal thread of the latter. 




and digesting food for the whole colony. A very long,, 
exceedingly mobile tentacle (Fig. 14 h) is attached 
close by the base of each nutritive polyp. This is be- 
set with many finer tentacles of a secondary order 
each of which bears a battery of thread-cells, of very 
complex structure. The thread-cells, of which each 
battery contains several hundreds, are delicate micro- 
scopic poison darts, beset with hooks, and connected 
with a poison vesicle. They produce a burning sen- 
sation in the human skin like that due to stinging- 
nettles. With these terrific lethal weapons the long 
tentacle on the look-out for prey fishes ever in the 
water around, ready in a moment to embrace the un- 
suspecting victim that approaches it, and to pierce it 
with thousands of deadly poisoned darts. In Anthe- 
modes (Fig. 14) the battery, densely filled with thread- 
cells, has the form of a spirally-twisted ribbon (Fig. 

A secondary tentacle (i) from Fig. 14 highly magnified, a. 
Point of attachment to the tentacle. I. Battery in the form 
of a spirally-rolled ribbon, h. Bell-shaped investment of 
the upper half. m. Terminal thread of the battery. 


Fig. 15. 


15 /), hidden in its upper half by a little bell (Fig. 
15 k\ extended into a fine terminal thread (m) below. 

With these terrible robber hordes harmless polyps 
are commingled in larger numbers. These represent 
the intelligence of the community. They have their 
internal and external body-layers adapted to testing 
and forming judgments, after the fashion of sense- 
organs. They feel, will, and think for the rest of the 

Fig. 16. 

A tactile or sense polyp of a compound Hydrozoon (Agal- 
mopsis). A knobbed tentacle is attached to its base. 




citizens of the state, amongst whom these mental func- 
tions are developed either far less strongly or not at 

Fig. 17. Fig. 18. 

Fi<*. 17. Male of a Compound Hydrozoon (Hippopodius.) a. 
Stalk by which the Medusa bell (b) is connected with the 



stem. Four radiating canals, connected at the margin by a 
circular vessel, traverse the disk, d Gastric cavity end- 
ing blindly, c. Spermatozoa, formed in the wall of the 

Fig. 18. Females of the same. References as in Fig. 17. 
Eight large eggs (c) are visible in the wall of the gastric 
cavity (d.) 

all. These sense polyps or tactile individuals (Fig, 9 t> 
Fig. 16) are like their nutritive fellows, but mouthless, 
and are provided with one long, delicate tentacle, en- 
dowed with very sensitive properties in place of the 
armed predatory tentacles. Finally, we find amongst 
all these different forms of individuals borne on the 
same stem the two sexual animals (Fig. 9 g) whose 
function is the reproduction of the whole colony. 
These are very generally placed in groups, like bunches 
of grapes, near a tactile polyp. Male (Fig. 17) and 
female (Fig. 18), very dissimilar in form, nevertheless, 
like the swimming motile polyps, are based upon the 
fundamental plan of the bell-shaped Medusae. In the 
wall of the gastric cavity of these sexual Medusae the 
reproductive cells are developed. The mouth is closed. 
The males are in general elongated, the females more 
rounded in shape. 

Different as all these various individuals of the Sipho- 
nophora community are in shape and in function, they 
none the less are all so intimately connected one with 
another that the older observers regarded the whole 
colony as a single individual, and the separate indi- 
viduals in the colony, the Medusae and the polyps, as 
organs of that individual. Certain individuals are 
hollow internally, and their cavities communicate 
freely with the cavity of the central stem, with the 
cavity, in short, of the chief polyp, on whom these 
others are placed. The nutritive fluid prepared by the 
feeding polyps is transmitted from them to the central 
stem-polyp, and thence is given out in portions to the 
other individuals of the community as from a central 
soup-kitchen. Each gets as much of this Spartan diet 
as its interior, i.e. the cavity of its body, can contain. 

Further, the intimate communal connexion of all the 
individuals shows itself also in the fact that a common 



volition animates the whole colony. After violent 
injury to one individual the pain is shared, as it were, 
with all the rest, causing the whole swimming com- 
munity to contract or to flee hastily away. In all this 
the voluntary movements of the citizens of this state 
are clearly co-ordinated. But, without any interference 
with the collective will of the state, each adult citizen 
has, to a certain extent, an individual will also, and 
can, if by accident or voluntarily it is separated from 
the rest, live an independent life for a long time. 

The strikingly different structure and function of the 
different individuals of the Siphonophora are entirely 
the result of division of labor carried out very com- 
pletely. All these various forms can be traced back 
to two fundamental ones ; the first, polyp-shaped, 
fashioned like the Hydra, and the second Medusiform, 
fashioned like the Aurelia. From the hydraf orm polyp 
originate by specialisation : 1. the central stem or 
central polyp with the pneumatophore (Fig. 9 a) ; 2. 
the nutritive polyps and their prehensile tentacles (Fig. 
14) ; 3. the tactile polyps with their sensory tentacles 
(Fig. 16.) On the other hand have arisen, by speciali- 
sation, from the Aurelia-like Medusae : 1. the swim- 
bells or motile polyps (Figs. 11, 12) ; 2. the bracts or 
protective individuals (Fig. 13) ; 3. the male Medusae 
(Fig. 17) ; 4. the female Medusa (Fig. 18.) And 
even the two fundamental forms, the Medusa and the 
hydra-polyp, are again developed primarily from an 
earlier primal polyp-form of the simplest nature. 

Not only does Comparative xlnatomy teach us with 
certainty that actually in the grey past, many millions 
of years ago, only simple polyps existed as representa- 
tives of the whole class of Hydromedusae, and that 
from these later on the simplest Medusae, and much 
later the connected Siphonophora colonies, evolved by 
gradual advances in the division of labor. The history 
of the development of the individual Hydromedusae 
teaches us the same truth, with the same certainty. 
For ontogeny, or the individual evolution of each 
organism, i.e. the series of forms through which it 
passes from the egg to the adult condition, is for us a 
replica in the briefest of time, and in broad, general 



outlines of its phylogeny, its ancestral history, its 
palaeontological evolution, i.e. in other words, the series 
of forms through which the ancestors of this organism 
have passed in succession since the appearance of 
organic bodies on the earth. 1 

If we now, keeping in mind this important con- 
nexion between ontogeny and phylogeny, between the 
evolution of the individual and of his ancestry, glance at 
the individual evolution of the Siphonophora, we find 
that from the impregnated egg of the Siphonophora 
colony nothing more than a very simple polyp develops. 
This elongates and becomes the central axis of the 
whole colony, giving rise by gemmation to all the 
other individuals, polyps and Medusae alike. At first, 
in the young bud-state, these are very similar and in- 
distinguishable. Presently and gradually each indi- 
vidual, increasing in size, takes on by division of labor 
its own special form. Of course, this division of labor, 
as it advances during the first few weeks in the course 
of the evolution of the egg, is at first acquired by 
inheritance from the ancestors. But this inherited 
division of labor of the Siphonophora community re- 
minds us, beyond a doubt, of the original specialisatior. 
arising from adaptation among the earlier Hydrome- 
dusas, a specialisation that has been evolved in the 
course of thousands of years by adaptation, by use, by 

The remarkable specialisation of the Siphonophora, 
the association of the differently constructed indi- 
viduals in a single state, whose citizens are connected 
not only intellectually but bodily, seems, perhaps, at 
first an extraordinary and mysterious phenomenon of 
nature. In reality a similar kind of specialisation is 

1 In the twelfth lecture of my " Natiirliche Schopfungs-geschichte " 
(p. 22f ), and in chapter xxiii. of my " Generelle Morphologie " (vol. 
ii., p. 371), I have dealt in detail with the exceedingly important causal 
nexus between ontogeny and phylogeny, i.e. the internal causal con- 
nexion between the evolution of every organic individual and the evo- 
lution of the series of its ancestors since the commencement of organic 
life on the globe (a connexion that, in consequence of the action and 
reaction of the laws of heredity and of transmission, is of necessity 
mechanically conditioned). 



very widely spread. In truth, all the higher plants 
present us with something of the same kind, for every 
branching flowering plant, every blossoming tree, 
ever} r flower-stalk, is in all essentials composed after 
the manner of the Siphonophora colony. The vegetable 
individual that corresponds with the single polyp or the 
single Medusa is the shoot, i.e., every branch, every 
independent axis bearing leaves. A flowering plant is 
made up of just as many individuals as it has branches 
and twigs or independent axes. Some of these indi- 
viduals bear green leaves only, and are concerned in 
the nutrition of the plant, like the nutritive polyps. 
Others form many-colored flowers, with stamens and 
carpels, and are concerned in reproduction, like the 
two kinds of sexual Medusae of the Siphonophora 
colony. In the case of the flowering plant also the 
distinction between the two individuals, the nutritive 
leaf shoots and the reproductive flowering shoots is not 
noticeable at first. It is acquired later on by speciali- 
sation. 1 

But with these the widest domain of specialisation is 
by no means exhausted. Comparative Anatomy and 
the study of Evolution teach us rather that its influence 
reaches much further afield. Every animal and vege- 
table individual, whether it lives solitary, like the un- 
branched plants and most animals, or is associated with 
its fellows in a community, like the Siphonophora and 
the majority of plants, every individual is again made 
up of many similar and dissimilar parts. These parts 
or organs subserve in their far-reaching specialisation 
those co-ordinated functions of the organism that we 
name in a single word, " life." Life is no mysterious 
product of a mystical life-force, but the totality of the 
functions of different organs marked off one from 
another by specialisation. The unity of organism in 
the individual in its narrower sense, i.e. the person, 
arises by the co-ordination and specialisation of the 
organs, as the unity of the colony or of the state 

1 Alexander Braun, in his admirable " Betrachtungen iiber die Er- 
scheinung der Verjiingung in der Natur " (Leipzig), has dealt in detail 
with specialisation in plants. 



arises by the co-ordination and specialisation of the 
persons. 1 

In the vegetable kingdom all the different forms of 
the nutritive leaf -bearing shoots and of the reproduc- 
tive flower-bearing shoots arise by specialisation of two 
simple fundamental organs, the leaf and the stem or 
axis. These two organs again have arisen by speciali- 
sation from one common, original, fundamental organ, 
the thallus. In like manner in the Arthropoda — the 
Insecta, Myriapoda, Arachnida, Crustacea — all the 
various jointed appendages of the body, the antennae, 
mandibles, maxillae, maxillipedes, and the true ambu- 
latory limbs have originated by specialisation from one 
and the same original fundamental form of simple limb, 
from a primitive appendage. 

What is the origin of these primitive or fundamental 
organs, that form by their continued specialisation all 
the various parts of the body, and by their co-ordina- 
tion the complex organism of the " person." These 
simplest fundamental organs are in their turn again 
the complex product that results from the combination 
into an aggregate, and from the specialisation of very 
many small organic individuals. These elementary 
units, that can generally only be distinguished by aid 
of a microscope, are generally known as cells. All 
organisms, all plants and animals are composed of many 
cells, except the very simplest, such as the Monera and 
those that are in composition only a single cell. The 
apparent unity of life of every- multicellular organism 
is, like the political unity of every human state, the 
compounded resultant of the connexion, and of the 
specialisation of these little citizens. They are the 
veritable elemental organisms, or the individuals of 

1 To explain clearly the immeasurable significance of the principle 
of specialisation of organs in the formation of the more highly deve- 
loped co-ordinated animal body, the person, it would be necessary to 
go through the whole account of the structure of the various orga- 
nisms. As, however, this inquiry, interesting as difficult, would carry 
us much too far at present, I must refer the reader for its details to 
the third book of my " Generelle Morphologic" In this I have worked 
out both the relation between the physiological and the morpholo- 
gical entities and the six different stages of the organic individual : 
1. Plastids ; 2. Organs ; 3. Antimeres ; 4. Metameres ; 5. Persons • 



the first order. 1 The organic cells may, in consequence 
of adaptation to the life conditions of their environ- 
ment, assume forms the most diverse. But the original 
cell-form, whence all others are later on evolved by 
specialisation, is a small mass of jelly-like matter, a 
minute particle of albuminoid semi-fluid matter, the 
material of the cell or protoplasm. This little body, 
generally, but not always, invested by an external enve- 
lope, the cell-wall or membrane, contains a smaller body 
more solid, and also albuminoid, the cell-nucleus. But 
even these two most essential constituents of every 
cell, the outer cell-substance and the inner nucleus, are 
not distinct one from the other in the simplest and 
most primitive of all organisms, the Monera and other 
Protista. They have risen originally from the exceed- 
ingly simple and homogeneous protoplasmic structures 
of these beings by specialisation of the minute, invisible 
albuminoid particles, the plasm-molecules or plastidules. 

Every cell in the plant and the animal body has, to 
a certain extent, its own independent life. It feeds and 
grows on its own account. It multiplies by reproduc- 
tion for the most part by fission. Even the function of 
performing movements is originally the property of the 
•cell substance of all cells. But it is generally limited 
to the withdrawal and enclosure of the cell within a 
self-made prison, within a rigid capsule or cell-mem- 
brane. Finally, every cell has a certain amount of 
irritability or sensibility, that in the most perfect of 
all cells, those of the animal brain, rises to the height 
of self-consciousness. 2 

<\. Communities. See further my essay on "Die Individualist des 
Thierkorpers." (Jena, Zeitschr. fiir Naturw., 1878.) 

1 In reality the individuals of the first order are generally called 
plastids. Besides the true nucleated cells the non-nucleated cytods 
come under this category. On this see my " Plastiden Theorie " (in 
the Biological Studies) ; also the thirteenth lecture of my " Natiir- 
liche Schopfungs-geschichte " (p. 286), and the " Generelle Morpho- 
logic " (vol. i., p. 269). 

2 The cells, or in a more general sense, the plastids (i.e. the cells 
and cytods), are the actually living units, the elemental individuals, 
and the forms and functions of the multicellular organism are the 
resultant, whose components are the form, connexion, function of 
all the cells associated in that organism. This cell-theory, so highly 
important to the mechanical or scientific conception of life (in a 



That specialisation of the cells, or cell metamorphosis, 
which must be regarded as one of the first and most 
important causes of the endless variety in organisation, 
is far more marked in the animal than in the plant 
kingdom. If the body of a higher animal, i.e., a dog, 
is resolved, by aid of the microscope, into its elemental 
constituents, an extraordinary number of different 
kinds of cells are met with in the various organs. The 
hair, cuticle, claws of the dog are made up of many 
different horny kinds of cells, that have all originated 
by specialisation from one common kind of epidermal 
cell (Fig. 19.) The skeleton, that forms with its bones, 

Fig. 19. 

B. A small piece of the cuticle, made up of flat, angular, epi- 
dermal cells. Each cell encloses its round nucleus. (Highly 

cartilages, tendons, and ligaments the solid framework 
of the whole canine body, consists, again, of different 
kinds of bone-cells; cartilage-cells, connective-tissue- 
cells, that have all originated from a single primal 
species of connective-tissue-cell (Fig. 20). The red 
flesh or muscle that covers the bones and executes 
voluntary movements is made up of very elongated 
and transversely striped cells (Fig. 21). The pale yel- 
low muscles, on the other hand, that enter into the 

broader sense, this plastid theory), is grasped by no one so thoroughly 
as by Rudolph Virchow. It has been applied more thoroughly by 
him, especially in relation to the human organism, than by any one, 
and his " Cellular-Pathologie " laid the foundation of a new epoch in 
scientific medicine. See also his excellent essay " Ueber die Einheits- 
bestrebungen in der wissentschaftlichen Medicin " (Gesammelte 
Abhandlungen, Frankfurt, 1856), and " Vier Reden liber Leben und 
Kranksein," Berlin, 18G2, especially the second discourse, " Atome 
und Individuen/' 


Fig. 20. 

A small piece of bone, presenting nine stellate bone corpuscles, 
connected by radiating offshoots, and lying imbedded in the 
osseous ground substance. (Highly magnified). 


Fig. 21. 

Three transversely striped muscle fibres (a) with several fat 
cells (b) lying behind them. 



Fig. 22 

A soul-cell or ganglion-cell from the brain of an electric fish 
(Torpedo). In the middle of the large ramified cell lies the 
nucleus, enclosing a nucleolus and most internally a nucleo- 
linus. The protoplasm of the cell is traversed by many very 
delicate fibrilla3. The poles or extensions of the branching 
cell go partly to nerve-threads (a), serve partly (b) for put- 
ting this particular soul-cell into relationship with others. 

walls of the stomach and effect the involuntary move- 
ments of this organ, are made up of smooth, unstriated, 
fusiform cells. Finally, the nervous system, highest 
set of organs in the animal body, subservient to sensa- 
tion, will, thought, consciousness, in a word, to the so- 

Fig. 23. 

The ovum of man, very strongly magnified. The ovum-cell 
(l-5th inch in diameter) is surrounded by a finely striated 
yolk-membrane, and contains a clear germinal vesicle with 
a dark germinal spot. 



called soul-functions or spiritual life, is composed of 
large stellate cells, of soul-cells, whose ramifying ex- 
tensions are connected with the nerve-fibres or delicate 
albuminoid fibres, extending from the cells (Fig. 22). 

Different in kind as are the above-named cells, that 
on microscopic investigation wc find interwoven one 
with the other, all have but evolved by specialisation 
from a single primitive form of cell, viz., from that 
homogeneous, simplest cell to which the egg, at the 
commencement of the animal's development, gives 
origin. Every animal is at the commencement of its 
individual existence a simple ovum (Fig. 23). But 
this egg is in its turn a simple cell, consisting of the 
same essential parts as every other cell of the gela- 
tinous cell-substance (here called yolk), and the in- 
cluded cell-nucleus, which in the ovum is called the 
germinal vesicle. Frequently, but not always, the 
animal ovum is enveloped by a special investment, the 
yolk-membrane (Fig. 23). 

A B . _ r D 

Fig. 24. 

Segmentation or continued fission of the egg at the com- 
mencement of development. The ovum divides at first into 
two cells (a), then into four (b), eight (c), and finally into 
many cells (d). 

As soon as the ovum of the dog or of any other mam- 
mal begins to evolve into a new individual, it divides 
by fission into two similar halves (Fig. 24 A). Without 
doubt the germinal vesicle or nucleus divides first, and 
then the investing cell-substance, the yolk. Each of 
the two daughter-cells thus produced divides again 
into two cells (Fig. 24 B). From these four, eight are 



soon formed by continuous fission, from the eight, six- 
teen ; from the sixteen, thirty-two, etc. Hence is 
formed at last from the simple egg or ovum a globular 
heap of very many and very small cells, that resembles 
a blackberry or a mulberry — the mulberry mass or 
Morula (Fig. 24 D). 

At first all these many cells are alike in form and in 
size ; soon, however, they begin to think of organising 
themselves into a state. They behave like a number 
of colonists who wish to found a well-ordered state, and 
betake themselves accordingly to the work necessary to 
that end. At first the cells of the Morula group them- 
selves into two chief sets, separating later by a species 
of foliation into two layers, lying one under the other, 
and known as primitive layers, or layers of the blasto- 
derm. The exceedingly important form called Gas- 
trula (Fig. 25) consists of these two primitive layers 

Fig. 25. 

JB. Gastrula of a mammal (rabbit). The whole body (shown 
in vertical section) consists of ninety-six cells, i.e., sixty- 
four clearer, smaller, ectoderm cells (e) and thirty-two 
darker and larger cells of the endoderm (i). These last fill 
up the gastric cavity (d) and oral aperture of the Gas- 
trula (o). 

alone. The exterior layer, epidermal, ectoderm (e)> fur- 
nishes the animalfcells for the organs of sensation and 
movement, skin, nervous system, etc. The inner la$ r er 



-epithelial, endoderm, furnishes the vegetative cells for 
the organs of digestion, nutrition, and breathing : in- 
testine, lungs, heart, etc. 

Later on the division of labor among these cells goes 
still further. The first set of cells undertake the pro- ^ 
tection of the animal organism and form the cuticle, 
hair, nails, claws. A second set form the solid frame- 
work of the body, as they are transformed into the 
<3ells of bone, cartilage, connective tissue. A third 
group extend into the long transversely-striated fibres 
that make up the fiesh or muscles, and by virtue of their 
special contractile faculty effect the movements of the 
different parts of the body. Finally, a fourth group of 
€ells, the most specialised, the most highly endowed \J 
cells, form the nervous system, and take on the highest 
functions of the animal body, those of volition, sensation, 
intellect (Fig. 22). Thus the various organs that com- 
pose the adult animal body arise solely by continued 
multiplication, connexion, specialisation of the cells. 
By specialisation of these organs, again, is produced the 
complex machinery of the composite organism that we 
see in each individual animal. 

The specialisation of cells and organs, as it can be 
traced out step by step in the evolution of every indi- 
vidual is, of course, not due solely to the adaptation of 
the animal to the conditions of existence of the sur- 
rounding outer world. It is rather transferred from 
the parents and ancestors of the animal under con- 
sideration by heredity. Of these inherited specialisa- 
tions of cells and organs the same thing is true that 
was said above as to the inherited specialisation in the 
Siphonophora. It leads us back to that original division 
of labor in ancestral forms acquired by direct adapta- 
tion, and developing itself slowly during many millions 
of years under the pressure of external conditions of 
life in the struggle for existence. That which holds 
in regard to the development of the whole vegetable or 
animal organism, holds also as to the development of 
all its individual organs and cells. The development 
of each separate cell (the ontogeny of the cell) repeats 
in swift succession and in broad outlines the history of 
the upbuilding of its ancestors (the phylogeny of these- 




cells). Therefore, from the simple fact that every 
animal is developed from a single simple cell, and 
from the manner in which this is brought about by 
specialisation of cells and of organs, we can draw this 
immense conclusion, that the earliest ancestral forms 
common to all animals were very simple cells, and that 
from the descendants of these simplest unicellular 
animals, by the collocation land continued specialisation 
of the cells, the higher multicellular forms of animals 
have evolved. 1 

At the end of a lecture that has only traversed a 
small region of the immeasurable fields of division of 
labor, it will probably be found that I have dealt very 
unequally with the two parts of the subject proposed ; 
that I have said much of specialisation in Nature 
generally, very little of it in human life. I must, 
therefore, make confession that I have been indulging 
in a harmless deception. For in the latter half of the 
lecture at least I have withal spoken ever of man, even 
though I named him not. For all that I have said of 
the composition of the body (and especially that of the 
dog) as of cells, all that I have said of the division of 
labor amongst the cells and organs of that body is true, 
word for word, of man. Our own body, like that of 
every higher animal, is an organised state, built up of 
millions of little citizens, the cells. These citizens 
lead to some extent an independent life. They form 
in their division of labor different ranks and classes of 
workers : such are the nervous system, the muscular 
system of our body, and so forth. The unity of life of 
the human individual, visible to outward eyes as the 
simple outcome of a personality, is, in truth, a highly 
complex resultant, compounded of the collective func- 

1 I have, in my "Naturliche Schopfungs-geschichte," shown hypo 
thetically how we can form an approximate mental conception of the 
historical evolution of all the different forms of animals, and generally 
of all organisms, from common ancestors of a very simple nature, first 
from the Monera (non-nucleated cytods), next from the simple 
nucleated cells up to the objects known to us at the present day. The 
fifteenth lecture, on the genealogy and history of the Protista ; the 
sixteenth, on the vegetable kingdom ; the seventeenth, on the inverte- 
brate animals ; and the eighteenth, on the Vertebrata, attempt a sketch 
of tin Evolution. 



tions of all those little citizens, the cells, and of the 
organs composed of these in specialised forms. If any 
of these citizens perform their duties imperfectly, or ^ 
become unfit for work, we fall ill, and if the unified, 
regular co-working of all, essential to life, comes to an 
end, we die. 

Moreover, that which I have said as to the history of 
the development of animals, and have ill istrated by 
the dog as an example, all holds, word for word, as to 
the development of man. Every human being is at 
the outset of his career, like every lower animal, a 
simple cell, an egg (Fig 23) ; and as this cell begins to 
develop, its daughter-cells and their descendants have 
to solve exactly the same problems in regard to division 
of labor as those already described in the develop- 
ment of the dog. The earlier stages of development of 
the egg of the dog, represented in Figs. 23 — 25, are an 
exact equivalent of those with which the individual life 
of every one of us has begun. 

In man also, as in his lower fellows, the manifold 
series of forms through which the organism passes 
during its individual development from the egg, pre- 
sents us, approximately and in outline, with a picture 
of the series of forms through which its ancestors have 
passed in the course of measureless ages. And this is 
a clear proof : first, that our race has evolved in relation 
to, in connexion with, lower organisms, and in most 
intimate connexion with the Yertebrata ; second, that 
our most ancient common forefathers had but the 
structure of a very simple cell. But the mighty law 
of Nature, under which from an origin thus simple 
have evolved all the forms, endless in number and in 
variety, of the animal world, and at its head the 
different races of man, so far surpassing all other 
beings, is the great law of division of labor or of spe- 




THERE is no subject in the whole range of man's 
knowledge on which, in former times and even 
to-day, men's views have differed so widely as the 
subject of the existence of the soul. What is the soul ? 
Whence comes it ? Whither goes it ? Has man alone 
a soul, or do other animals also possess one ? And 
where in the animal kingdom are to be found the 
limits, the beginnings of existence of the soul ? We 
encounter even to-day these and kindred questions, as 
we have encountered them for one or two thousand 
years, without receiving an answer that has met with 
general scientific approval. 

This persistent obscurity, as to one of the most im- 
portant and most difficult questions in the whole 
range of human knowledge shows itself in nothing so 
clearly as in the circumstance that psychology, even at 
the present time, occupies a very vague position amongst 
the rest of the sciences. The majority of scientific 
men at present regard the soul-activity of man and 
other animals as a real phenomenon of nature, and 
therefore believe that only through the investigations 
of science is there any possibility of lightening the 
darkness that overhangs the subject. On the other 
side the majority of the psychologists range themselves. 
These, appealing to the list of philosophers who have 
dealt with the history of the soul, take the opposite 
view, and look upon the existence of the soul — at least 
in man — as a supernatural, a spiritual phenomenon, 
conditioned by forces altogether different from ordi- 
nary physical forces, and that mocks, in consequence, 
all explanation that is simply scientific. According to 
the opinion of these men — an opinion still largely pre- 
valent — psychology is in part or in whole a spiritual 
science, not a physical one. 


Despite the opinion so widely spread, so influential,, 
despite the distrust with which every scientific man 
enters upon the subject of the soul, a subject involved 
in obscurity, we will none the less venture on the 
attempt to penetrate these very mysteries by aid of the 
light of scientific method. We find at once an invita- 
tion to this inquiry, and a warrant for it, in two funda- 
mental facts. First, in every living being the soul 
undergoes a consecutive development. The soul has 
its developmental history. Secondly, a part at least 
of the soul -functions is connected with definite body 
organs, and is not conceivable save in connexion 
with those organs. This part, at least, of soul-phseno- 
mena is immediately reachable by scientific investiga- 
tion. Further, the fact is now generally admitted that 
at least parts of the soul-functions, especially will and 
sensation, are performed in the higher animals in the 
same fashion as in man ; and a physiological compari- 
son of different animals shows us a long scale of vary- 
ing grades of development in regard to the souls of 
animals. Hence it is for the zoologist, who makes the 
investigation of animal life in its every phase his life- 
work, not only a privilege, but a duty, to study the 
origin and limits of soul-existence in the animal king- 

Without doubt the untrodden path upon which the 
zoologist thus enters is very different from the high- 
way along which the multitude of psychologists have 
serenely ambled. It is well known that these thinkers 
hold self-consideration, observation, and reflexion on 
the human soul alone as the most important, frequently 
as their only end and aim. Hence the soul, as gene- 
rally analysed and described in the text-books on psy- 
chology, is the soul of man alone — is, for the most part, 
the highly-cultured soul of a philosopher learned and 
skilled in thought. Assuredly the accurate knowledge 
of a soul thus highly-cultured is of greatest value ; but 
it never touches many of the momentous questions of 
dur knowledge ; it is wanting in that very particular on 
which the science of to-day rightly lays the greatest 
stress, the knowledge of development. 

Development, long, gradual, ascending, is at the basis 


of the soul in each individual man, as in every other 
animal. That is a psychological fact of fundamental 
significance. The greatest thinkers of all ages, Aristotle 
and Plato, Spinoza and Kant, have once on a time 
been children ; their mighty universal minds have 
been evolved by infinite ascending stages. The zoolo- 
gist who turns his attention to the investigation of the 
soul is compelled by these facts to use, more than all 
other means, that most important method of inquiry, 
the study of development. He will work out the com- 
parative evolution of the soul in man and in other 
animals, and will examine the comparative anatomy 
and evolution of those organs of the body which in 
animals as in man, the highest of animals, are con- 
cerned directly in the functions of the soul. The com- 
parative morphology of the soul-organs, and the com- 
parative physiology of the soul-functions, both founded 
on Evolution, thus become the psychological problem 
of the scientific man. 


The first, most general, and most important fact that 
the scientific student encounters at the very outset of 
his psychological inquiries is the dependence of all 
functions of the soul on certain known material parts 
of the animal body, called organs. In man and in the 
higher animals such organs occur — the sense-organs, 
the nervous system, the muscular system. In the 
lower animals we meet with groups of cells, or even 
individual cells, which have not yet differentiated into 
nerve and muscle. Every expression of the soul-life, 
every psychical working, is indissolubly connected 
with such an organ, and is not thinkable without it. 
Therefore nothing should be said on the subject of the 
existence of the soul unless we recognise the way in 
which the ^vxrj (Psyche or soul) is bound up with its 
organs. But it is not superfluous to lay stress on this 
fundamental physiological fact at a time when the 
most crass superstition, in the shape of Spiritualism, 
raises once again its head, and we see not only many 
thousands of civilised and uncivilised men, but even 



scientific men of repute and knowledge, falling in 
blind frenzy at the altar of this superstition. 

Within the last few months, to our shame be it said, 
we have seen that the American Spiritualist, Slade, 
after he had acquired in England considerable wealth 
by his juggling with the " spirits," and at last had been 
unmasked as a bare-faced impostor, continued his 
swindling trade in Germany with the like success. He 
was even able to befool not a few scientific men of 
some repute. And do we not also see that a special 
literature of Spiritualism, represented by numerous 
periodicals, aims at dressing up this incredible swindle 
in the garb of special science ? In the century of 
railroads and telegraphs, of spectrum analysis and of 
Darwinism, in the age of the monistic conception of 
Nature, such reversions to the dark superstitions of the 
middle ages seem scarcely conceivable. They are only 
to be explained by reference to the "mystic nightside" 
of the human soul, that fatal inclination towards the 
supernatural and the mystic, which religious supersti- 
tion has most carefully fostered these thousand years. 
It is certain that this tendency to mysticism takes root 
in this ineradicable fashion because it is strong with 
the bequest of a thousand years, and has been con- 
tinually strengthened and sanctified anew by pretended 
revelations, i.e., by mental phenomena of a pathological 

In opposition to all those pretended spiritual mani- 
festations of Spiritualism, which, like the miracles of 
Louise Lateau, or the Madonna of Marpingen, are 
founded partly upon unconscious illusion, partly upon 
conscious fraud, one clear physiological fact stands 
firm as first essential of all knowledge of the soul. 
That fact is, that every kind of soul-function is in- 
separably connected with certain parts, or organs, of the 
body. Our first duty, therefore, must be to make our- 
selves somewhat better acquainted with these organs. 
The organs of our soul-life — viz. : 1. the sense organs ; 
2. the nervous system ; 3. the muscles — constitute one 
large apparatus, that we designate in a word as the soul- 
apparatus. In man, as in all the higher animals, this 
armory of the (mental) functions shows us a marvellous 



congeries of the most complex organs and tissues, and 
without doubt the complexity is greater and more 
intricate the higher and more perfect the working of 
the apparatus, that is, the higher and more perfect the 
working of the soul (Fig. 26). 

A voyage of discovery in this wonderful labyrinth is 
most attractive, most instructive, but it is at the same 
time very difficult. Instead of this, it will answer our 
purpose much better to look at the far more simply 
constructed apparatus met with in some lower animal. 
We choose, therefore, a lowly-developed worm, not so 
much because man, according to Faust, " is like the 
worm that crawls in the dust," nor because phylogeny 
to-day, in dealing with man's genealogy, tells us of a 
series of worms among our ancestors, but rather because 
the lower worms show their soul-organs as structures 
very simple and very easily seen ; thus they facilitate 
in most excellent fashion the difficult study of the 
highly complex organs of higher animals. 
' Let us study under the microscope such a simple 
worm, e.g., a Rotifer or a Turbellarian (Fig. 27). We 
see in front and above the mouth a small white swell- 
ing, whence fine cords radiate in all directions to the 
different parts of the body. That white swelling above 
the mouth is a soft mass of nerve-tissue, and is the 
centre of the whole soul-apparatus, a brain of the 
simplest kind. The fine cords, which radiate from the 
brain to all regions of the body, are nerves. We dis- 
tinguish two different sorts of nerve-fibres. Some are 
the organs of the will, motor nerves, or nerves of 
motion. They run from the brain to the muscles, 
whose fibres, the muscle-fibres, are caused by the motor- 
nerves to contract. The others, on the contrary, are 
instruments of sensation, or sensory nerves. They 
carry the different sense-impressions from the external 
skin and the sense-organs to the brain, and thus bring 
that organ into relation with the environing external 
world. The sense-organs of so low a worm as this we 
study are of course still very simple, but for that very 
reason they are deeply interesting. In many worms 
the external integument plays the part of a general 
sense-organ, and responds to sensations of different 





kinds, especially alterations of pressure and changes of 
temperature. In other worms are added to this general 
sense-organ, the skin, special feelers or tentacles, and 
in addition eyes of the simplest kind, dark specks in 
the skin, enclosing a refracting lens. Ears, also, of the 
simplest kind, appear, viz., a pair of depressions or 
vesicles in the skin, which are lined with minute 
delicate hairs ; auditory cilia, that are thrown into 
vibration by sound-waves in a special manner. 

Fig. 27. 

Nervous system of a Turbellarian. Two kinds of nerves radiate 
from the simple nerve-ganglion, or brain (g) : the centri- 
petal sensory nerves (s) pass to the integument (h), the 
tentacles (£), the auditory vesicle (o\ the eyes (a), : the 
centrifugal motor nerves (m) pass to the flesh, to the sub- 
integumental muscular layer (/). w . Cilia on the inte- 

It is a fact of the greatest significance that these 
organs of the higher senses, eyes and ears, in the lower 



worms are nothing more than a specially developed 
part of the external integument. For the far more 
highly developed and more perfect eyes and ears of the 
higher animals and of man originate in the most ex- 
ternal layer of the integument, and do not clash with 
the law only recently discovered, and of such vast 
importance — the law of the development of all sense- 
organs from the skin. All the different sense-organs of 
an animal are originally only specialised parts of its 
sensitive external integument. 

But further, the organs of movement, those servants 
of the will, called muscles, are originally in closest 
relationship to the outer skin. 
s In our low worm the whole muscular system is 

Fig. 28. 

Ovum-cell of a worm. The globular cell-body (6), consisting 
of protoplasm, is surrounded by a delicate membrane (a), 
and contains a cell-nucleus (c) and uncleolus (cl). 

Fig. 29. Fig. b'O. 


Fig. 31. Fig. 32. 

Division of the ovum or segmentation at the beginning of 
development. From the simple cell (Fig. 28) arise by re- 
peated division, first two segments (Fig. 29), then four (Fig. 
30), eight, sixteen (Fig. 31), finally many cells (Fig. 32). 
The mulberry-like, multicellular, globular body or Morula 
thus formed (Fig. 32) passes later into the cup-shaped Gras- 
trula (Fig. 33). (See my 44 Anthropogenie," 3rd edition, 
1877, chapter viii.) 

Gastrula of a calcareous sponge (Olynthus). Fig. 33 (a) ex- 
ternal view. Fig. 34 (b) ]ongitudinal section through the 
axis. i. Inner cellular layer or mucous layer (endoderm). 


e. Outer cellular layer or epidermal layer (ectoderm). The 
two layers surround the gastric cavity (g) of the animal, 
that opens externally by the mouth (o). 

Fig. 36. 

Fig. 37. 

Fig. 38. 

Longitudinal sections 
through the Gastrulae of 
animals belonging to five 
different classes. Fig. 35 
Vermes (Sagitta b). Fig. 
36. Echinodermata (Ura- 
ster, c). Fig. 37. Crusta- 
cea (Nauplius, d). Fig. 38. 
Gasteropoda (Linnseus,E). 
Fig. 39. Vertebrata (Am- 
phioxus, f). In all, e is 
the epidermal or outer 
layer of the blastoderm. 
i. Mucous or internal 
layer, d. Gastric cavity. 
o. Oral aperture. 

Fig. 39. 

represented by a thin layer of muscle, that extends 
continuously beneath the skin (Fig. 28). Generally, 
this " dermo-muscular layer " of worms is divided into 
two distinct layers, an outer layer of circular fibres and 
an inner layer of longitudinal fibres, although it is not 
yet differentiated into distinct groups or muscular bands, 
as in the higher animals. 

We must call especial attention to the fact that all the 
nerves, both the centripetal sensory fibres, which run 
from the brain to the skin and the sense-organs, and 


the motor fibres, which run from the brain to the 
muscles, are in immediate connexion with these peri- 
pheral parts. If, agreeably to nature, we conceive all 
the soul-apparatus as one united whole, the sensory 
sense-organs are nothing more than special terminal 
expansions of the sensory nerves, the voluntary mus- 
cular fibres are nothing more than special end-organs 
of the motor nerves. The brain is intercalated between 
these two sets of organs, as a central point of junction, 
as a direct connecting structure. 

If we wish to obtain a clear idea of the functions of 
such an apparatus, of the nature of soul-life, the familial* 
comparison with an electric-telegraph system is of 
greatest service. Not only does this well-known com- 
parison hold throughout all the structure of the soul- 
apparatus, but in the performance of its functions ; in 
truth, electric currents play a part of the greatest im- 
portance. But the simile has its full significance only 
when we by aid of microscope have studied the very 
fine structural elements of the nervous system. The 
microscopic structural elements of the soul-apparatus 
are none other than those of which the other organs of 
the animal body are composed — the so-called cells. 
Here, as everywhere in natural history, it is that cell- 
theory of Schwann and Schleiden, enunciated forty 
years ago, which opens to us, as with a master-key, the 
principal door that leads to more accurate knowledge. 
Diverse as are the innumerable forms of small cells in 
the different tissues of plants and animals, yet all agree 
in the main fact, that every single cell has a certain 
degree of individual independence, has its own shape, 
lives its own life. As Briicke has put it in a notable 
phrase, every microscopic cell is an elementary orga- 
nism, " an individual of the first rank." Nay, as we 
shall soon see, we must even ascribe to every cell an 
independent soul — a cell-soul. 

Countless as the stars in heaven are the endless 
myriads of cells which compose the frame of a whale 
or an elephant, of an oak or a palm. And yet the 
gigantic body of either of these most gigantic organisms, 
like the invisible minute form of the smallest of orga- 
nisms, at the commencement of its existence only con- 




sisted of one small cell, the ovum, invisible to the naked 
eye (Fig. 28). This cell begins to develop, and from it 
arises, in a very short time by repeated division, a large 
mass of similar cells (Figs. 29 — 32). These arrange 
themselves in leaf-like layers, one within the other, the 
so-called germ-layers, or layers of the blastoderm. At 
first all the cells are alike : each individual cell is of 
the very simplest form and composition : a round, 
white, albuminoid spherule or small mass of protoplasm 
that encloses a solid nucleus. But soon dissimilarities 
and differences appear ; the cells begin to take part in 
the work of life and assume different shapes and 
qualities. Alimentary-canal-cells undertake digestion, 
blood-cells the transformation of material, lung-cells 
respiration, liver-cells the secretion of bile. Further, 
the muscle-cells dedicate themselves exclusively to 
movement, sense-cells to the various sensations : the 
tactile cells of the skin learn to recognise alterations of 
pressure and of temperature, the hearing-cells learn to 
distinguish sound-waves, the visual cells to distinguish 
light- waves. But the nerve-cells enter upon a career 
at once the most difficult and the most splendid, and 
amongst these it is the brain-cells that gain, as in a 
noble strife, the highest prize, and as soul-cells rise 
high above all other kinds of cell. 

This division of labor in the cells, or, as the anato- 
mist calls it, tissue-formation, so full of meaning, is 
performed in a short time in the individual develop- 
ment of every animal and of every plant under our 
very eyes. It commences with the development of the 
animal from the egg, even at that early time when the 
progeny of the ovum, the so-called embryo-cells (Fig. 
32) separate into layers. The embryo has at this time 
the form of a double-walled cup, and the two walls of 
this cup or of the Gastrula (Figs. 33, 34) are the two 
primitive layers of the blastoderm. From the inner or 
mucous layer (endoderm, Fig. 34 t) the organs of nutri- 
tion and digestion, the organs of the vegetative func- 
tions, develop. From the outer layer of the blastoderm, 
the epidermal layer, or sense-layer (ectoderm, Fig. 34 e), 
arise the organs of the animal functions, muscles and 
nerves, skin and sense-organs — in a word, the soul- 



organs. We must call attention to the fact, as of greatest 
significance, that in all multicellular animals, from the 
Hydra to man, the specialisation of the cells begins on 
this wise, with the separation of the two primitive 
layers of the blastoderm, and that the soul-apparatus 
always takes origin from the cells of the ectoderm. In 
animals of every class, nerves, sense-organs, muscles, 
arise from the sense or epidermal layer of the Gastrula 
(Figs. 35—39). 

The tissue-formation, that we see take place under 
the microscope with astonishing rapidity, is only a 
repetition in brief, conditioned by heredity, of a long 
and often-repeated historic process, a replica of an 
ancestral process which has taken millions of years to 
arrive at the present condition, and during which the 
specialisation of the cells, in the most exact sense of 
the phrase, by adaptation to the different life-functions 
of the cells, has gradually been evolved in the struggle 
for existence. The cells, therefore, behave like the 
cultured citizens of a well-organised, civilised country. "~ 
In fact, our own life, like that of all the higher animals, 
is exactly such a civilised cell-state. The so-called 
tissues of the body, muscle, nerve, glands, bone, con- 
nective tissue, etc., answer to the different ranks or r ! j K ^ 
bodies of the state, or still more accurately to the heredi- 
tary castes that we meet with in ancient Egypt, and 
even at the present day in India. The tissues are 
hereditary cell-castes in the civilised state of the multi- 
cellular organism. But the organs, which are composed 
on the other hand of different tissues, may be compared 
to the various offices and institutions. At the summit 
of all is the mighty central director, the nerve-centre, 
the brain. The higher the development of the animal 
the more complete is the centralisation of the cell- 
monarchy, the mightier is the directing brain, and the 
more grand is the construction of the electric-tele- 
graph apparatus of the nervous system, that brings the 
brain into connexion with its most important subjects, 
the muscles and the sense-organs. 

In comparison with all this, the arrangement of the 
soul-apparatus in our worm, mentioned above, is very 
simple, though not different in essentials. If we irri- 




tate the animal in any way — if we touch its delicate 
integument with the point of a needle or with a piece 
of ice, the changes of impression or of temperature 
resulting from these stimuli are instantly perceived by 
the sensitive cells of the skin, that as sentinels guard 
at all points the layer of the skin ; they, in contact with 
the exterior, telegraph at once their perceptions through 
the nerves of the skin to the brain. In like fashion 
the waves of sound that strike upon the auditory vesicle 
are perceived by the auditory cells of the latter as 
noises or as musical sounds, and are notified by the 
auditory nerves to the brain in telegraphic fashion. 
Not less swiftly the visual cells of the eye, which are 
impinged upon by a ray of light, send a telegram of 
light or color to the brain. Here is placed the head 
government of the cell-state, consisting of a few great 
stellate cells whose radiating extensions are in im- 
mediate connexion on the one hand with the nerves of 
sense causing sensation, on the other with the motor- 
nerves that excite movement. As soon as a telegram 
from the sense-nerves as to any change in the sur- 
rounding world has arrived at the central office, this 
information is communicated from the first brain-cells 
or ganglion-cells that receive the stimulus to others, 
and the chief counsellor now determines what is to be 
done. The result of this determination is telegraphed 
as will through the motor- nerves to the muscles, that at 
once obey the command sent to them by the contraction 
of their fibre-cells, by motion. 

Without doubt the nerve-cells of the brain, the 
ganglion-cells, or soul-cells (Fig. 40), with their radi- 
ating poles, connected one with another by a branching 
net-work, play the most important part in the life of the 
soul. They form, in fact, the central directing organ of the 
whole of the multicellular body. They gather together 
all the reports from the external world that are sent to 
the brain via, the centripetal telegraph-wires or sensory 
nerves. They convey, in like manner, all the orders 
of the will that pass out to the muscles by the centri- 
fugal paths of the motor nerves. And, in addition, 
these notable soul-cells of the brain effect that most 
important and enigmatical work that we denote by the 



Fig. 40. 

v A soul- cell or ganglion- cell from the brain of an electric fish 
(Torpedo). In the middle of the large ramified cell lies the 
nucleus, enclosing a nucleolus and most internally a nucleo- 
linus. The protoplasm of the cell is traversed by many very 
delicate fibrillas. The poles or extensions of the branching 
cell go partly to nerve-threads (a), serve partly (b) for put- 
ting this particular soul- cell into relationship with others. 

word Ideation. They, in the higher animals, as in 
man, effect that most exalted of all functions of the 
soul — that of thinking and of perception, reason and 

Whilst we are now dealing with the loftiest regions, 
and the noblest workings of soul, life, reason, and con- 
sciousness, we must further state that in truth the 
exact nature of these difficult cell-functions is as yet 
wholly unknown, but that we are in a position, by the 
aid of comparative physiology and the history of 
development, to throw a clear light on this intricate 
question. For, in the first place, the study of the souls 
of animals reveals to us a long series of evolution, in 
which every conceivable grade of reason and of con- 
sciousness is represented, from beings wholly without 
reason to those whose reason is most highly developed, 
from sponges and polyps to the dog and the elephant. 
In the second place, we see in every child, and in the 
earlier condition of all higher animals, that reason and 
conscience are non-existent at birth, and that they 
develop only slowly and gradually. In the third place, 
we regard it as a fact that a hard-and-fast line between 
unconsciousness and conscious soul-functions is as little 
existent as between irrational and rational thinking, 
that these opposed functions rather touch one another 
at innumerable points without presenting any defined 
limits — that, in short, they glide one into the other. 

It is well known that the obscure question of the 
nature of consciousness plays a chief part in the psycho- 
logical contests of the present day. The celebrated 
physiologist, Du Bois-Reymond, has pointed out, in his 
expression " Ignorabimus " (we shall not know), to the 
scientific congress at Leipzig, that consciousness is an 
entirely insoluble problem ; a boundary of our know- 



ledge that the mind of man, even in its most expanded 
development, will never pass. Many others consider 
consciousness as a special prerogative of man, wanting 
altogether in other animals. This latter opinion 
assuredly no one will share who has observed steadily 
and attentively the conscious and carefully considered 
actions of the dog and the horse, of bees, ants, 
and other reasoning animals. But the first opinion 
also is not tenable. Attentive observation of ourselves 
teaches us that our innumerable conscious and uncon- 
scious actions glide continually one into the other. 
Numberless acts of our daily life, e.g.,, the use of cup, 
knife, fork, reading and writing, the playing of musical 
instruments, and the like, depend upon complicated 
functions of nerve and muscle that have to be learnt 
originally by careful and conscious attention, but by 
degrees have grown unconscious from use and habit. 
Every morning, when we wash and dress, rise up and 
go out, we perform quite unconsciously hundreds of 
complex movements that originally had to be con- 
sciously learnt, with pains and by degrees. On the 
other hand, these various unconscious acts at once come 
into distinct consciousness as soon as, from any reason, 
our attention is directed to them, and our self-observa- 
tion is aroused. As soon as we make a false step on 
the stairs, or notice a false note on the pianoforte, we 
at once become conscious of the unconscious actions. 
Besides, we can also unerringly trace out step by step 
the gradual development of consciousness in every 
child. Resting on these facts, w r e no longer doubt, 
therefore, that consciousness depends upon a complex 
function of the soul-cells, which was at first gradually 
acquired through adaptation, and through the inherit- 
ance of recent adaptations has been slowly extended. 
The comparative study of development of the soul-life 
in the animal kingdom teaches us the same truth. 
The complex molecular motions in the protoplasm of 
the soul-cells, whose highest consequence is imagina- 
tion and thinking, reason and consciousness, have been 
gradually acquired, in the course of many millions of 
years, by Selection. The brain also, organ of these 
functions, has developed in the course of these long 



periods, at first slowly and gradually, from the simplest 
to the most perfect form. And in this, as ever, the 
evolution of the organ goes hand in hand with that of 
its function : the implement improves with use. 

For confirmation of this view, from which such 
important consequences follow, the comparative obser- 
vation of the nervous systems of different anirilals is 
of the greatest moment. The simple brain of the worm, 
with the nerve-cords radiating thence, has been the 
starting-point for many different sorts of complex 
arrangements in the nervous systems of the higher ani- 
mals. These latter bear the same relation to the early 
forms of the same order as the grand telegraphic system 
of the Germany of to-day, with its hundreds of stations 
and thousands of workers, bears to the first simple 
model of an electric telegraph by means of which the 
discoverer, forty years ago, introduced one of the most 
momentous improvements in the exchange of thought 
between the nations. The more highly-developed the 
sensation, volition, intellect in an animal, the more 
complex and the more centralised is the arrangement 
of the soul-apparatus that does this psychological work, 
the more prominently does the nerve-centre assert 
itself ; for upon it depends the unity of direction of 
the whole system. 

We are for the most part accustomed, therefore, to 
name the centre of the nervous system, the brain, in its 
w r ider sense, as the seat of the soul. Yet in truth this 
common expression is inaccurate, and we can only 
grant it a figurative meaning in a kindred sense to that 
in which we speak of an able woman as " the soul of 
the house," an all-powerful minister as " the soul of 
the State." As we will not deny to others their indi- 
vidual soul, so we dare not deny that the soul exists in 
the millions of cells that constitute the nervous appa- 
ratus of the higher animals, whose brain we therefore 
name the seat of the soul. When in the Franco-Prus- 
sian war, in 1871, Paris, that is in truth the soul of 
centralised France, and, according to Victor Hugo, is 
even the soul of the world, was rigidly blockaded by our 
victorious army, when telegraphic communication with 
the rest of France was wholly cut off, in the severed 



limbs of France nevertheless the ramified, network of 
the telegraphic system worked away unceasingly, and 
the dauntless soul of Gambetta organised new armies 
for the relief of the beleaguered capital. In like 
manner physiological experiments on decapitated frogs 
and insects teach us that despite the removal of the 
brain the soul-life can last for a long time in other 
parts of the body. The unifying central director of 
the whole alone is destroyed ; only the highest soul- 
functions, reason and consciousness, are thereby in 
part or as a whole annihilated ; the other functions con- 
tinue. If we place a drop of corrosive acid on the skin 
of the headless frog he wipes it off just as skilfully as 
if he were still possessed of a head. If we hold a 
headless beetle fast by one leg, he tries to escape with 
the five other legs as quickly and cleverly as if he had 
not lost his brain at all. Sense-function and feeling, 
will and muscular movement, will last a long time after 
the brain is removed. With the brain only unifying 
consciousne3s and centralisation are lost. We are 
bound, therefore, to distinguish clearly between these 
conscious central souls of the multicellular animal and 
the separate souls of its innumerable cells ; the latter, 
indeed, are subordinated to the former, but are always 
to a certain extent independent. 

The organ of the central soul is the collection of 
soul-cells, the ganglionic cells of the brain ; on the 
other hand, the organ of the single cell-souls is the 
very body of the cell itself, protoplasm and nucleus or 
some part of these. 


Perhaps no class of animals is, after the Mammalia, 
of such importance in the comparison of the lower and 
higher stages of development of the soul-life as the 
class Insecta. For although the numberless different 
sorts of insects only furnish endless variations of a 
single original theme, although recent study of pedi- 
grees derives all butterflies and beetles, flies and bees, 
Orthoptera and Neuroptera, from one common stock, 
nevertheless the variations in development of their soul- 
functions are very extraordinary. The familiar con- 



trasts between the stupid goose and the hawk, acute of 
sense, between the dull rhinoceros and the clever 
elephant, seem insignificant in comparison with the 
immense contrasts furnished by the soul-functions of 
the various insects. On the one hand are many lower 
insects, e.g., plant-lice, the cochineal insect, bugs, and 
generally parasitic insects of different kinds. These 
occupy a very low position in Evolution, not higher 
than that of the majority of the Vermes. Their one 
desire is to eat and drink. On the other hand the 
higher insects, and especially the social insects, the 
bees, wasps, ants, white ants, with their complex states, 
rise to a height of mental function that can only be 
compared with that of the civilised peoples who form 
like states. A wondrous division of labor, especially 
among the ants, leads to the organisation of their state 
into different ranks, whose members are distinguished 
by special marks and peculiarities. Among them we 
distinguish not only males and females, but also soldiers 
and workers, peasants and artisans, governors and 
slaves. Their operations of agriculture and gardening 
are not limited to the laborious collecting of stores and 
preserving of fruits. They rise to the height of actual 
cultivation of vegetables and the careful breeding of 
plant-lice, their cows, whose sweet honey-fluid they 
suck. No less marvellous is the architectural skill of 
the ants and Termites, that is shown in the plan of 
their grand palaces with their thousand rooms and 
chambers, corridors and steps, doors and windows. In 
addition to these arts of peace, we call to mind their 
skill in the rude art of war, whilst the strategic ability 
with which armies of ants engaged in warfare try to 
surround and enclose one another day after day, shows 
clearly that they also are children of this iron nine- 
teenth century. In some South American species, 
indeed, as consequence of excessive military exercises, 
an exclusively military spirit has arisen, which has led 
to the abandonment of the early peaceful occupations, 
and to a robber life, as of Circassian hordes. Finally, 
we remember how even that institution of human 
civilisation, slavery, has been in vogue among the ants 
longer than in our own highly-civilised feudal race. 



There are ant-colonies that carry on formal slave-hunts, 
steal from other species their young and carry them off 
as veritable slaves : nay, these slaves, in fact, later on, 
repudiating all ties of nature, regard the advantage of 
their cruel masters as of more importance than that of 
their own race, and actually help to steal fresh bands 
of slaves in the predatory expeditions. Although these 
highly interesting facts in regard to the mental life of 
ants have been discovered by Huber and other ento- 
mologists for more than a hundred years, they were 
held for a long time as fabulous products of the fancy. 
But of late the numerous inquiries of more recent 
times have thoroughly confirmed the old discoveries, 
and added to them new and more extensive observa- 

Certainly the intellectual contrasts between the 
shrewd ants and their stupid cows, the plant-lice, are 
more marked than the immense distance between the 
god-like genius of a Goethe or a Shakspere and the 
poor soul of a Hottentot or an Australian aborigine. 
And yet, in the one case as in the other, a long series 
of connecting links presents itself between the two 
extreme terms of the series. None the less the fount 
of origin of them all is the same. As the majority of 
men derive our race from one common ancestor, so all 
zoologists with one accord hold firmly that all the 
different orders of insects spring from one common 
ancestral insect. Consequently the very different soul- 
functions of these same insects must have been gra- 
dually evolved by adaptation to varying conditions of 
life, and by continued inheritance have grown to the 
so-called Instincts. 

No idea has given rise to so many errors and mis- 
understandings in the comparative study of soul as 
this so-called " Instinct." As long as the older natural 
history held that all the different kinds of animals with 
their special natures arose by a supernatural act of 
creation, it was obliged to hold in like manner, that 
the specific soul-function of each species had been 
created once for all, and that thus every step in 
the life of the animal had been immutably determined 
beforehand. The totality of the instincts, which on 



this view ought to determine, unalterably, unfailingly, 
the nature of the actions of any species of animal, 
amongst which the most noteworthy are the so-called 
artistic instincts of the nesting birds and of the bees — - 
this sum of instincts was considered as a primarily 
created thing. This wide-spread view has become 
altogether untenable since we know, thanks to Darwin, 
that neither individual animals are created as such nor 
are their special instincts invariable. We now know 
that all species of a given class of animals take origin 
originally from one common stem, and that, like their 
other qualities, their instinct undergoes change and 
transformation under the mighty influence of Natural 
Selection. When animals are placed in new unusual 
conditions of life they adapt themselves to these, new 
thoughts arise, they make new inventions, acquire new 
instincts. Necessity is the mother of invention, and 
practice makes perfect. The hard fight for existence 
makes at all times and at all places such powerful 
demands on the instinct of self-preservation of animals 
that they are compelled, even as is man, to learn and 
to labor. It is not true, though it is frequently asserted 
to-day, that the beaver builds its water palace, the 
swallow its nest, the bee its honey-comb, in the same 
fashion in all ages and in all countries, the same to-day 
as it has been these 2,800 years. Further, we know 
through reliable observations that these highly de- 
veloped artistic instincts undergo very remarkable 
changes, and adapt themselves to the conditions of 
individual localities. The last of the Mohicans in the 
shape of beavers that live in certain parts of Germany 
at the present day, have adapted themselves to the 
police regulations of civilisation, and no longer build 
those magnificent water palaces after the manner of 
their forefathers during the last 2,000 years. Whilst 
the cuckoo with us in Europe lays its eggs in a strange 
nest it has not acquired this bad habit in America. 
Every experienced bee-fancier knows how the special 
habits of bees are often lost in individual hives. It is 
a well-known fact that nightingales, finches, and other 
birds of song learn new melodies, master by the process 
of imitation new successions of notes, and therefore 



change their musical instinct. And do we not see 
plainly, in our house dogs, hunting dogs, badger dogs, 
sheep dogs, that instincts new and different have been 
learnt by education, by use, by habit ? 

Comparative experimental inquiry, unprejudiced, 
free from bias, renders it certain that the " instinct " 
of animals is no other than a summary of soul-functions 
which have been originally acquired by adaptation, 
strengthened by habit, and handed down from genera- 
tion to generation by inheritance. Many instinctive 
acts of animals, originally performed consciously 
and with reflexion, have in course of time become 
unconscious, after the manner of all the accustomed 
acts of reasoning in man. We might with equal pro- 
priety consider these last as expressions of an innate 
instinct, for unconsciousness often occurs in connexion 
with the " instinct " of self-preservation or of maternal 
love and with social instincts. Consequently neither 
is instinct an exclusive quality of animal brains, nor is 
reason a special prerogative of man. Rather is it that 
in the unbiassed comparative study of souls we find a ; 
long, long series of gradations, of gradual improve- * 
ments and higher developments in soul-life, which 
pass down from higher to lower men, from perfect 
to imperfect animals, step by step, even to that simple 
worm whose single nerve-ganglion is the starting- 
point for all the numberless forms of brain in the vast 

As, in fact, no break occurs anywhere in this series, and 
as in the simple soul-apparatus of our worm are already 
contained all the structural elements — nerves, sense- 
organs, muscles, out of which are constructed, in most 
complex fashion, the marvellous soul-apparatus of ant 
and man — scientific men, by common agreement at the 
present hour, hold that in all these animals that are 
provided with a nerve-apparatus a soul-life or a soul 


But what is to be said of those lower animals in 
which a nervous system, even of the simplest nature, 
is wanting — the corals, polyps, sponges ? Does the 



want of a nervous system in these denote the lowest 
limit of soul-life ? Or is there in these cases a soul 
without any nerves ? Well-known scientific men — e.g., 
B. Yirchow and Du Bois-Reymond — answer the latter 
question in the negative, and maintain that no one can 
speak of a real soul-life in these nerveless animals. 
We are of a different opinion, and we found it on the 
general opinion of all zoologists who have employed 
themselves for a long time and with great perseverance 
in close observations on these animals destitute of 
nerves. Nay, we are convinced that these animals, 
without nerves and yet alive, are actually of the deepest 

Fig. 41. 

Two fresh-water polyps (Hydrae), one on the left contracted, 
one on the right expanded : the latter bears a bud that has 
already seven short prehensile tentacles. 


moment in comparative physiology, and yield us the 
true key to the comprehension of soul-development. 

The member of these important classes of lower 
animals most instructive, best known, and most closely 
studied, is the common fresh-water polyp, the Hydra 
(Fig. 41). This delicate little being, only a millimetre 
long, is diffused everywhere in our lakes and ponds, 
and can be had at any time in large quantities. Few 
would anticipate the wealth of disclosures of an im- 
portant nature that this insignificant being yields in 
regard to the most important mysteries of life. Its 
simple body has the form of an elongated cup, colored 
sometimes grey or green, sometimes brown or red. 
The cavity of the cup is the stomach of the Hydra, its 
opening is the mouth. Around the mouth is placed a 
circle of fine threads, 4 — 8 in number, that serve both 
as tactile organs and as prehensile structures for the 
seizing of food. We look in vain for eyes and ears, 
muscles and nerves in our Hydra, and yet we are certain 
that it is sensitive and motile. If we touch the slender 
outstretched body but gently with the point of a needle, 
it contracts instantly into a round globular mass (Fig. 
41, to the left). If we place a tumbler containing Hydra? 
in the window, in a few hours all the polyps are 
gathered together on the side of the glass nearest the 
light. They are sensitive to light though they have 
no eyes. They crawl towards the light although they 
have no muscles. Sensation and definite movement, 
therefore, the most important signs of the soul-life 
in animals, are present, beyond a doubt, despite the 
fact that the special organs of the soul, muscles and 
nerves, are wanting in them. How is this enigma to 
be solved ? Have we here a function without a corre- 
sponding organ, soul without soul-apparatus ? 

The microscope gives a decisive answer to this 
question. The cup-shaped body of the Hydra in 
reality is made up of two cups of similar shape 
placed one within the other, their walls everywhere 
in close apposition. Essentially it is a Gastrula. If 
we now observe the thin double wall of the Hydra 
body in fine sections under a high power, we see 
that each of the two cups is composed of a special 


layer of cells. These two cell-layers have altogether 
different nature and significance. The cells of the 
inner layer effect exclusively the vegetable func- 
tions of nutrition, digestion, metastasis. The cells of 
the outer layer, on the other hand, perform the animal 
functions of sensation and motion. If we tease out 
this external layer with needles, we see on many of the 
cells thus isolated one or more long thread-like exten- 

Three neuromuscular cells of Hydra. The outer nucleated 
part is sensitive, nervous (n) : the inner filamentous part is 
motile, muscular (m). 

sions (Fig. 42). Close investigation shows that these 
fine threads run in a circular direction between the 
two walls of the cup-shaped body, and, like a muscle, 
effect the contraction of those walls, whilst the external 
rounded nucleolar part of the same cell is sensory. 
We are here encountered by the noteworthy and deeply 
momentous fact that a single cell effects in its solitary self 
the most important functions of the soul ; the external 
rounded part of the cell, sensation, the internal filamen- 
tary part, the will and spontaneous movement. The 
outer half of the cell is nerve, the inner muscle ; with 
rigid accuracy, therefore, the discoverer of these struc- 
tures, Kleinenberg, names these soul-cells of Hydra 
"neuro-muscular cells." The whole of the soul-appa- 
ratus of our polyp consists of nothing more than a 
single simple layer of such neuro-muscular cells, and 
each one of these cells does in most simple fashion that 
which the complex soul-apparatus of higher animals, 
with its different nerve-, muscle-, and sense-cells does 
in a more perfect manner. But in this case, as might 
be expected, a central apparatus or brain is wholly 

Fig. 42. 


wanting. Instead of a brain the whole external layer 
of the body is in our minute polyp the seat of the soul. 
We therefore cease to wonder at those astounding capa- 
bilities of division of the Hydra that have been known, 
thanks to the experiments of Trembley, since 1744. 
If we cut a fresh-water polyp up into fifty little pieces, 
within a few weeks as many complete polyps are 
developed from the fragments. Every portion of the 
cup-shaped body grows straightway into a complete 
animal. The cell-souls of all the individual neuro- 
muscular cells are equally complete. 

Fig. 43. 

A Medusa (Eucope). The stomach, whence pass four nutri- 
tive canals to the margin of the umbrella, hangs in the 
middle and upper part of the bell-shaped body. In the 
middle of the canals lie the eggs (g). From the margin of 
the umbrella (b) depend four tentacles, and between these 
are eight auditory vesicles (a). 




The neuro-muscular cells of the Hydra are also, to» 
borrow a housewifely phrase, " maids of all work." 
Each individual in the soul-economy of this little polyp 
effects all the various labors that are shared in the 
higher animals among the muscle-, nerve- and sense- 
cells of different kinds. All the latter sorts of cell, 
differing so widely one from another, have therefore 
arisen by division of labor from simple neuro-muscular 

The umbrella-shaped sea-bells, stinging-fish or Me- 
dusae, present us with the first result of this division of 
labor. These animals are closely related to the Hydra- 
polyps, but are much more highly developed (Fig. 43). 
Any one who has spent a few weeks on the sea-shore 
will certainly have seen now and again fleets of these 
beautiful, bell-shaped, jelly-like animals, and any one 
who has come, when bathing, into unpleasant contact 
with them will remember the disagreeable burning 
sensation that resulted, as if from the touching of a 
stinging-nettle. The order to which the Medusae belongs 
is therefore called Acalephae. If by means of a glass ves- 
sel we cautiously remove such a Medusa from the sea, 
and investigate more closely its structure, we find pecu- 
liar soul-organs. On the margin of its umbrella-shaped 
body are true eyes of a simple order and ear- vesicles,, 
and nerves put into connexion sense-cells and muscle- 
cells. The latter effect the powerful swimming move- 
ments of the Medusa. But here also muscles and 
nerves are in most intimate relation with their place of 
origin, the external skin, and a specialised brain or 
central organ of the whole soul-apparatus is still 

Compared with the simple, minute, stationary Hydra, 
the gigantic, beautiful, free-swimming Medusa appears 
to us, beyond doubt, a much higher and more perfect 
animal. Nevertheless these two beings, placed by man 
formerly in. different classes, are in the closest con- 
nexion, for in the course of its history the Medusa- 
form has evolved from the Hydra-form. Nay, even to- 
day the majority of the Medusae take origin directly 
from polyps. From the stomach-wall of the little 
sea-polyp, kinsman of the Hydra (Fig. 44), a bud de- 


velops, which gradually becomes a Medusa, and later 
falling off like the ripe fruit of a tree, swims freely 
hither and thither. But from the ova of this Medusa 
no Medusae arise, but polyps, buds (Fig. 45), that fix 
themselves and grow up into the Hydraform cup. 
From this well-known "alternation of generations" 
result in regular succession two animal forms, widely 
differing one from another. The great grandmother 
resembles the mother, the grandmother resembles the 
daughter, but the two series are very unlike one 
another. The first, third, fifth, seventh generations 

Fig. 44. 

Three Hydraform polyps (Corymorpha) fixed to the sea-bottom. 
Two of them are putting forth Medusiform buds (Steen- 
strupia), three of which have already become detached. 

are minute, lowly-organised, fixed polyps (Fig. 44) ; 
the second, fourth, sixth, eighth, on the other hand, 
are represented by large, more highly organised, free- 
swimming Medusae (Fig. 45). And, what is more 
interesting to us in this connexion, the latter have 

M 2 



nerves, muscles, sense-organs, the former have in place 
of these a delicate membrane, consisting of a layer of 
neuro-muscular cells. Both generations have souls, 
both possess will and feeling. But, not unnaturally, 
the simple lower soul-life of the polyp does not rise to 
the height of the Medusa-soul ; the latter has evolved 
from the former and long after the former in the course 
of time. 

Fig. 45. 

A Medusa (Steenstrupia) formed by gemmation from the 
polyps of Fig. 44. The stomach, whence four canals run to 
the circumference of the disk, depends from the middle of 
the bell. On the circumference are placed four eyes, but 
only one long tentacle. 



In yet another significance is the notable class of 
Hydro-medusae of deepest interest for the comparative 
study of the soul. For from it the Siphonophora have 
evolved, those swimming colonies of animals that are 
of extraordinary importance in the study of the phy- 
siological division of labor. The Siphonophora are 
found swimming about on the smooth surface of warm 
seas, but only at certain times, and not in large 
numbers. They belong to the inexhaustibly rich 
wonder- world of Nature, and whoever has once had 
the good fortune to observe for any length of time 
living Siphonophora will never forget the glorious 
spectacle of their marvellous forms and movements. 
Such a Siphonophoron is best compared to a swimming 
flower-stem, whose variegated leaves, blossoms, fruits, 
are exquisitely shaped, delicately colored, and as if 
built up of polished crystal (Fig. 46). Every indi- 
vidual flower-like or fruit-like element of the swim- 
ming stem is really a Medusa individual, i.e., a 
Medusiform animal. But the different Medusae of the 
colony have, as result of division of labor, assumed 
altogether different forms. Part of these Medusae 
attend simply to the swimming (m), another set to 
nutrition and digestion (n), a third to sensation (£), a 
fourth to offence and defence, a fifth to egg-formation 
(g). Those various vital functions that every ordinary 
single Medusa performs for itself, are here shared 
amongst the different individuals of the colony. These 
latter have modified their bodily structure in accord- 
ance with their peculiar individual functions. 

Just as in the ant-kingdom, so in the Siphonophora 
kingdom many animals of various form but of one 
kind are connected in a lofty social community. But 
whilst in the much higher ant-state the ideal band of 
social interests and the feeling of duty to the state 
binds together the free citizens, in the Siphonophora- 
state the individual members of the community are 
directly forged together into a bodily whole, bound as 
slaves to the yoke of the chain of state. Here, in truth, 
every single individual has its own personal soul ; if 
separated from the stock it can move and feel inde- 
pendently. But in addition the whole stock has a 



single central will upon which the individual polyps 
are dependent, and a general sensibility which com- 

Fig, 46. 

A Siphonophoron (Physophora) swimming in the sea. a. Pneuma- 
tophore or swimming bladder at its upper end. m. Nectoca- 
lyces or swimming bells, o. Opening of the umbrella. 
t Tactile polyps, g. Egg-forming female individuals, n. 
Nutritive individuals. 



municates every perception of one individual instantly 
to all the rest. Each of these Medusa-beings of the 
Siphonophora stock can say with Faust : " Two souls 
dwell, alas, in my breast." The egoistic soul of the 
individual lives in compromise with the social soul of 
the stock or state. 

Woe to that Medusa of the colony who wishes, in 
fatuous egoism, to free itself from the general com- 
munity and to live a free life on its own account ! 
Unable to perform all the separate functions that are 
necessary to its existence, the functions performed for 
it by its different fellow-citizens, separated from these 
last, it soon comes to grief. For one Medusa of the 
colony can only swim, a second only feel, a third only 
eat, a fourth only seize prey and ward off foes. The 
harmonious co-operation and the reciprocal help of all 
the individuals of these swimming brotherhoods, the 
common feeling, the central soul that connects them all 
one with another in a true affection — these alone give 
the life of the individuals and the life of the great 
whole lasting stability. In like manner the true fulfil- 
ment of all civic and social duties on the part of the citi- 
zen constitutes the stability of human civilised states. 


The most important fact with respect to our inquiry 
into the soul that we gain from the observation of these 
remarkable Siphonophora is the conviction, full of 
meaning, that the single soul of an apparently simple 
animal can be made up in actuality of many different 
souls. The unity of the soul is so pronounced in the 
delicate sensations and free movements of the Sipho- 
nophora, that the earlier zoologists regarded the whole 
colony as a single simple animal, as an individual, and 
this inaccurate opinion still has supporters of some 
eminence. Dissections and the study of development 
carried on by men free from prejudice, easily convince 
us that the apparently simple soul in these animals is 
in truth but the sum of the conjoined single souls. 
Strange as the fact appears at first, we find something 
akin to this in all social animals, and even in man. 
Do we not speak of the heart of the people, of the 



feeling of the community, of a national will ? Do we 
not see in a thousand historical instances the way in 
which this heart of the people, this national spirit,, 
feels as one, and thinks, wills, acts as a single man ? 
As one man, a whole nation rises from beneath the 
oppression of a cruel despot, breaking the throne of the 
tyrant into fragments. As one man a wronged nation 
feels the disgrace of wounded honor, and takes ven- 
geance on the insulter. When for 1,400 years the re- 
sistless flood of migrating tribes overflowed all Europe,, 
when in like irresistible fashion in the year 1848 all 
the nations of Europe won for themselves new free- 
ways for their political development ; in such moments 
of the world's history as these the single might of an 
idea, of a distinct form of thought, in its vast fulness 
meets us face to face. And yet this apparent unity of 
idea is in reality the sum of many thousand isolated 
ideas that have risen in the individual souls of all the 
citizens, or at least of an overwhelming majority, all 
struggling towards the same goal. 

As with the soul-life of the nation on the large scale, 
so is it with the spiritual life of the individual man 
and of the higher animals generally on the small scale.- 
For here also to the far-reaching glance of the zoologist 
the apparent unity of the soul merges into the indivi- 
dual cell-souls, the separate soul-functions of the 
countless cells of which the whole multicellular orga- 
nism is composed. Of course we could designate the 
cells of the brain of man and higher animals as " soul- 
cells " in a more restricted sense, in that they represent 
very especially the unity of the cell-state, and perform 
the unifying direction of that state. Still we must 
not on that account forget that this lordship over the 
functioning soul-cells has been acquired through wide- 
reaching specialisation and centralisation, and that, 
despite this fact, the special soul-life of every single 
cell of every other tissue still endures. Each blood- 
corpuscle, each cell of bone or skin, retains its own 
independent method of feeling and volition, until at a 
certain point it becomes subordinate in the highest 
issues to the all-pervading influence of the governing 



The soul of the cell is therefore a quite general, the 
cells of the soul, on the other hand, a quite special de- 
velopment of organic life. We are compelled finally 
to locate in every single living cell a soul ; special 
soul-cells, on the other hand, are only met with in the 
higher animals in a central nerve-system, and there 
perform unceasingly in loftier fashion those functions 
of the soul, which were originally performed by all the 
cells in a far simpler manner. Nevertheless these most 
highly-developed aristocratic soul-cells sprang primarily 
from simple cells of the lowest order, gifted with the 
non-specialised soul of the individual cell. 

Frankly, this our conception as to the cell-soul is not 
by any means generally accepted to-day. \ Indeed, it is 

Fig. 47. 

A unicellular Infusorium of the order Ciliata (ProtodohK 
a. Oral opening of the cell with funnel-shaped gullet, b. 
Contractile vesicle, c. Food pellets that have been ingested 
within the sarcode of the cell. d. Nucleus. Delicate hairs 
or cilia cover the whole surface of the cell, and serve both 
for sensation and voluntary motion. 

vigorously opposed at the present time by well-known 
authorities, such as B. Von Yirchow. But on the firm ' 
ground of the modern study of development as re- 
formed by Darwin, we are obliged to maintain that 
our theory of the cell-soul is a consequence, inevitable 
as momentous, of the uniform or monistic conception 



of Nature. I may be allowed, therefore, in conclusion, 
to glance briefly again at that lowest group of beings 
that seem to us as if specially designed for evidence as 
to the truth of these pregnant theories. 

Far down among the lowest stages of organic life, 
midway between the confines of the vegetable and 
animal kingdoms, connecting those kingdoms as by a 
bond, ebbs and flows that wonderful world of orga- 
nisms, microscopic, invisible to the unaided eye, that 
we name in general, animalcules, Infusoria, Protozoa, 
Protista. The large majority of these Protista remain 
their lives through in the form of a single simple cell. 
Yet it is beyond dispute that this cell possesses both 
sensation and real movement. Among the ciliated In- 
fusoria (Fig. 47) these soul-functions are exhibited so 
clearly in these strange beings that Ehrenberg, the 
celebrated investigator of the Infusoria, held firmly, 

Fig. 48. 

A creeping Amoeba, a simple, unicellular Protist that cease- 
lessly alters its shape, giving out extensions of its sarcode 
that are not persistent ; in the middle lies the cell-nucleus 
with its nucleolus. 

and in the most positive way, that in these animals 
nerves, muscles, brain, and sense-organs are to be 
found. Yet, in point of fact, no trace of these organs 
is visible. The protoplasm of the cell-body, the matter 
of the cell nucleus enclosed therein, these, and these 
alone, are in this case the material substratum of the 



soul-life, and form a soul-apparatus of the simplest 
kind. And if we but convince ourselves that there are 
even in these one-celled Infusoria widely different 
characters and temperaments, individuals clever and 
stupid, strong and weak, lively and dull, light-seeking, 
light-shunning, we can only realise the many gradations 
in the soul-life of these little beings by accepting as a 
fact the idea that fine differentiations obtain in their 
protoplasmic bodies. 

Amongst these unicellular Protista the so-called 
Amoebae are of special interest. These animals are to 
be seen under the microscope everywhere, in fresh 
water and in the sea (Fig. 48), The naked simple body 
has no definite shape. It alters its form continually 
in spontaneous fashion, as it stretches out, sometimes 
from this region, sometimes from that of its surface, 
a transitory finger-shaped process. These transitory 
pseudopodia, appearing and disappearing in ceaseless 
change, serve the creeping Amoeba both in definite 
motion, like feet, and in sensation, like tentacles. But 
further, many independent cells in the body of higher ani- 
mals are not perceptibly different from these Amoebae. 

Ovum of a calcareous sponge (Olynthus) that undergoes 
voluntary movements, and feels like an Amoeba, and, like 
the Amoeba, has a soul. 

As examples of these may be quoted the migratory 
motile cells. To these amoeboid cells belong, i.e., the 
lymph-corpuscles in our lymphatics, and the white 
blood-corpuscles of our blood that wander in myriads 
in the different parts of the body. The young ovum- 

Fig. 49. 



cells of animals are gifted in like fashion with definite 
motion and sensibility ; in many sponges these restless 
spirits wander round freely even in the bodies of their 
parents (Fig. 49). These ovum-cells, soul-gifted, are 
therefore of special significance, because all other cells 
of the organism originate from them. 

Soul-function in the wider sense is a general property 
of all organic cells. But if this is the case we cannot 
wholly deny to plants a soul-life. Further, the lowest 
plants are simple cells, and the body of all the higher 
plants, as of the higher animals, consists of countless 
individual cells. Only in the latter the division of 
labor and the centralisation of the state are much more 
fully carried out than in the former. The government 
of the animal body is a cell-monarchy, that of the 
plant a cell-republic. As all the separate cells in a 
plant remain much more independent than in an 
animal, the unity of the soul is much less possible in 
the former than it is in the latter. Only a few special 
plants, such as the delicate sensitive plant, the fly- 
catching Dionoea, offer exceptions to this rule. As a 
consequence the soul-life of plants is much less studied 
than that of animals, and but few scientific men have 
turned their attention thither. Amongst these, for 
example, must be named Professor Fechner, of Leipsic, 
the acute founder of the science of psychic physics, 
who has discussed the subject of plant-souls in a series 
of ingenious works. Besides the necessary acceptance 
of a plant-soul is also confirmed by the fact that we 
are not in a position to draw a sharp line of demarca- 
tion between the vegetable and animal kingdoms. 
The unicellular Infusoria or Protista form the bridge 
that unites the two great kingdoms of organic life into 
one vast whole. Only the gradation of soul-function 
is extraordinarily manifold, and differs widely in the 
two kingdoms. 

Amongst the most weighty advances in the new cell- 
theories ranks the knowledge that the most important 
substance of the cell, protoplasm, has throughout all 
living things the same essential nature, whether we 
study the unicellular Infusoria, isolated plant-cells, or 
a cell of an animal body. Its essential and most 



characteristic property is its vitality, or the power of 
the protoplasm to perceive stimuli of various kinds, 
and to respond to these stimuli by definite movements. 
That this property is common to the protoplasm of all 
cells without exception, we are at once convinced by 
microscopic observation. On the foundation of this 
oneness of the living protoplasm we base the hypo- 
thesis that the ultimate factors of soul-life are the 
plastidules, the invisible, homogeneous, elementary 
particles or molecules of protoplasm, that build up in 
endless variety the countless different cells. 

No reproach is more frequently made against the 
science of to-day, especially against its most hopeful 
branch, the study of development, than that it degrades 
living Nature to the level of a soulless mechanism, 
banishes from the w r orld the ideal, and kills all the 
poetry of existence. We believe that our unprejudiced, 
comparative, genetic study of soul-life gives the lie to 
that unjust accusation. For if our uniform or monistic 
conception of Nature is rightly founded, all living 
matter has a soul, and that most wondrous of all natural 
phaenomena that we usually designate by the word 
spirit or soul is a general property of living things. Far 
other than believing in a crude, soulless material, after 
the manner of our adversaries, we must rather suppose 
that the primal elements of soul-life, the simple forms 
of sensibility, pleasure, pain, the simple forms of motion, 
attraction, and repulsion, are in all living matter, in all 
protoplasm. But the grades of the up- building and 
composition of this soul vary in different living beings, 
and lead us gradually upwards from the quiescent cell- 
soul through a long series of ascending steps to the 
conscious and rational soul of man. 

Still less can we allow that the poetic and ideal con- 
ception of the universe is lessened or endangered by 
our monistic theory of development. Truly the nymphs 
and the naiads, dryads and oreads, with whom the 
streams and rivers of old Greece were alive, who 
peopled her woods and mountains, are wanting to us 
to-day. With the gods of Olympus they have long 
since passed away. But in the place of these anthro- 
poid demigods range the countless elemental spirits of 


the cells. And if any idea is poetic and accurate 
alike in highest degree assuredly it is the beautiful 
thought that in the smallest worms and in the most 
minute plants thousands of individual delicate souls 
are living ; that in every microscopic unicellular In- 
fusorium a special soul is busy, even as in the blood- 
corpuscles, ceaselessly travelling in our veins, or in the 
brain-cells that rise to the highest of all soul-functions, 
clear consciousness. From this point of view we see 
in the study of the cell-souls, the greatest step yet made 
towards the reconciliation of the ideal and the real 
contemplation of Nature, the old and the new concep- 
tion of the universe. 



FTTHE popular lectures on the subject of Evolution 
J- that form the second part of this series, have been 
printed without the introduction of any alterations. As 
to the reason for this series, and the grounds on which 
publication without alteration seem desirable, I refer 
to that which I wrote in the preface to the first part. 

The first of the five lectures in this second part is 
" On the progress and work of Zoology." This was 
delivered on January 12, 1869, in the university hall of 
Jena, w r hen I made my entry into the physiological 
faculty of our university. 

It has relation, in the first place, to my undertaking 
in 1865, the newly-founded chair of zoology. At my 
entrance into this chair I had at once the opportunity, 
the duty, the right, to discuss this subject. Nor does 
this discussion seem superfluous even to-day, if we 
think how diverse are men's views on the matter. For 
even skilled teachers of zoology at the present time call 
themselves professors of zoology and zootomy, and 
directors of institutes of comparative anatomy and 
zoology. As though zootomy and comparative anatomy 
were not parts of zoology ! It would be as absurd if a 
botanist called himself " Professor of botany and plant- 
anatomy." None the less this fact shows in striking 




fashion how little even to-day is the great scientific 
problem of zoology grasped even by its most famous 
interpreters. I believe that in my Anthropogenie, as 
well as in other places, I have shown, at sufficient 
length and in detail, how I have striven to work out 
this problem, and how I only look upon anthropology 
as a special branch of zoology. This lecture first ap- 
peared in print in the fifth volume of the " Jena Journal 
of Medicine and Science " (1870), and later as an in- 
troduction to the first part of my biological studies 
(Studies on the Monera and other Protista), which have 
been out of print for some time. 

The second lecture was delivered on November 19, 
1875, to the medical and scientific meeting at Jena, as 
an appendix to recent communications on the history 
of the Corals. (See my popular lecture on " Arabian 
Corals. A journey to the coral-reefs of the Red Sea, 
and a glance at the life of the coral builders." With 
7 colored plates and 20 woodcuts. Berlin, 1876). The 
facts of elementary development, there only hinted at, 
were worked out more fully in a pamphlet, appearing 
on May 9, 1876, under the title of " The Perigenesis of 
the Plastidule, or the Development of Life-Particles : an 
essay on the Mechanical Explanation of the Elemen- 
tary Stages of Development." This work was dedicated 
to the esteemed Curator of the University of Jena, Dr. 
Moritz Seebeck, on the jubilee or twenty-fifth year of 
his entrance on his honorable office. As a matter of 
course, the fundamental idea contained in this work, 
the regarding a diffused wave-movement of the mi- 
nutest animate particles or plastidules as the sole 
effective cause of the commencement of organic de- 



velopment, met with as little acceptance as the attempt 
made at the same time to explain, in a simple physio- 
logical way, heredity as the memory of the plastidule, 
variability as its power of comprehension. But though 
I see clearly enough the weakness of this hypothesis, it 
seems to me even now more reasonable and more in 
accord with the facts of our knowledge of cells, than 
the celebrated Theory of Pangenesis of Darwin. The 
essential antagonism between this hypothesis and mine 
of Perigenesis I have discussed in the course of the 
lecture. For this reason, and inasmuch as a third 
hypothesis on this subject does not, as far as I know, 
exist, I have thought it wise to reproduce the lecture 
here without alteration, if it were only for the purpose 
of arousing more skilled observers to its refutation by 
a better hypothesis. This much-vilified Perigenesis, 
like my genealogical trees, will probably result in the 
fault-finding becoming of less import than the sugges- 
tions of improvement. 

The third lecture, " On the proofs of Evolution," 
was delivered on March 3rd, 1876, to the same society 
as its predecessor, as an appendix to certain communi- 
cations on the structure of the Gastrula, and on the 
phylogenetic significance of the earlier ontogenetic 
stages, in which the difference between the primary 
Palingenesis and the secondary Cenogenesis is of 
especial moment. He who is interested in further 
particulars on this very important subject will find in my 
" Anthropogenie " (3rd edition, 1877, p. 9 et seq.) a more 
detailed account. The detailed scientific proofs are in 
my " Studies on the Gastrsea Theory " (Part ii. of the 
" Biological Studies," 1877, p. 61; "The significance 

N 2 



of Palingenesis and of Cenogenesis.") The lecture on 
the proofs of Evolution appeared for the first time in 
print in " Cosmos " (Vol. II., Part i., 1877). 

The fourth lecture, " On the Present Position of 
Evolution in relation to Science," was delivered on 
September 18th, 1877, at the first open sitting of the 
fiftieth meeting of German naturalists and physicians, 
at Munich. It appeared also in the official report, and 
independently in three large editions in September, 
October, November, 1877 (Stuttgart : Eduard Koch). 
Although this lecture, in which for the first time the 
introduction of the study of development into the 
school curriculum was proposed, is widely read, its 
place in this collection is justified, first as complement 
and supplement to the lecture given fourteen years 
earlier (1863), to the Stettin Association of Naturalists, 
who printed it in the first part of their collection ; 
second, because of the vigorous and general discussions 
connected with this Munich lecture. For in reply to 
it, four days later, Rudolph Yirchow, on September 
22nd, 1877, brought out his celebrated lecture, "On the 
Freedom of Science in its Modern Form," in which he 
made the strongest attack upon our earlier and upon 
our later theories of development. This lecture I 
answered in my work, " Free Science and Free Teach- 
ing " (Stuttgart : Eduard Koch, 1878). I then declined 
to enter further into the pedagogic side of the ques- 
tion. This side is, however, thoroughly investigated, 
and at the same time Virchow's arguments completely 
refuted, in the brief pamphlet, "Hypothesis in the 
Schools and the Study of Natural History " (Bonn : 
Emil Strauss, 1879). The author of this, a head-master 



In Lippstadt, Hermann Muller, is well-known as one 
of our ablest instructors, and [as one of those German 
scientists who, like his celebrated brother, Fritz Muller, 
in Brazil, have furthered with so much energy the 
Darwinian hypothesis by their own invaluable en- 
quiries. The violent attacks, the disgraceful calumnies 
to which Hermann Muller and Ernst Krause, the 
honored editor of " Cosmos " and author of " Werden 
nnd Vergeben," have been subjected of late in the 
Prussian Parliament, will make these excellent authors, 
as they ought to be, more generally known. 

Lastly, the fifth lecture, " On the Origin and De- 
velopment of the Sense-organs," was given on March 
25th, 1878, to the Scientific Club, at Vienna, and ap- 
peared, in October of the same year, in the third volume 
of " Cosmos " (pp. 20 and 99). It stands in close 
relation to the lecture, " Cell-Souls and Soul-Cells," 
given about the same time, and included in the first 
part of this series. 

Whilst I desire for this second part of my popular 
lectures a like kindly reception to that received by the 
first, I dare to hope that these lectures are carrying the 
clear light of Evolution into ever- widening circles, 
and arousing men to the grappling with these high 
problems of scientific inquiry. That light seems of 
tw T ofold value, that inquiry of twofold force, in a time 
like the present. On the one hand the dark clouds of 
political and intellectual reaction, ever threatening, are 
gathering. The attempt is being made to rob free 
inquiry, free teaching of the protection that is guaran- 
teed it by law. On the other hand, is rising a danger 
far worse, reaching far more widely, from those who 



more than all others are called upon to guard the right 
and duty of free knowledge. Inquirers after science 
of high name and fame, to whose freethinking en- 
deavors, to whose deep-thinking investigations, we 
were wont to pay highest honor, are quitting their 
sacred standard, are passing over into the camp of our 
deadliest foes. They do not simply propose the sub- 
jection of free reason to the yoke of the Church's 
blind dogma. They are not ashamed to cast them- 
selves into the arms of the grossest superstitions of 
the mediaeval times. For Spiritualism, already raising 
in threatening fashion its hydra-headed crest, is none 
other than this. It is not enough that many publica- 
tions are trying to clothe this gross fraud in the garb 
of true science. Certain naturalists of the foremost 
rank, as Wallace and Zollner, have let themselves be 
caught by the spiritualistic tricks of cunning pick- 
pockets, and are helping actually to the best of their 
power in the entrapment of men within the net of 
these refined deceivers. That a Friedrich Zollner 
should fall a victim to the trickery of a Slade is most 
lamentable, and this the more that the one has ren- 
dered to scientific criticism and the true study of 
Nature so many valuable services, and that the other 
has ere now been unmasked as a common cheat. 

Beyond these sad, these shameful facts, we cast a 
glance full of hope to the great mass of scientific men,, 
whose heads are free, whose hands are clean. Free 
Science, free inquiry, free teaching alone can ward 
off these threatening dangers, and as victors overthrow 
once again this superstition, the worst foe of human 
reason. And throughout the wide range of our know- 



ledge nothing can be appealed to for this end with 
such tremendous effect as our modern teaching of 


Jena, March 12, 1879. 



TO the academic professor entering into a faculty 
in customary way by a public oration, the most 
obvious and natural theme is a consideration of the 
scientific problems that he finds have to be solved in 
his own special calling, and the manner in which he 
intends to solve them. Such a discussion may seem 
trivial and unnecessary in the many branches of science 
that have long ere this acquired a definite direction, 
a clear end and aim, and as to whose subject-matter, 
extent, and treatment, more or less agreement prevails 
among those that teach them. On the other hand, such 
discussion would seem to be by no means without value 
in those studies that have not yet reached this stage 
of advancement, and are therefore thought of and 
treated in very different fashion. Of not one of the 
natural sciences dq,es this hold to such an extent as of 
zoology. I believe^ therefore, that I am doing nothing 
that is unnecessary when on my entrance into the 
philosophical faculty to-day I state my own conception 
of the aims of zoology at the present time, and discuss 
the manner in which I have endeavored to fill the 
chair that has been recently founded at Jena in this 

In the true understanding of such a demonstration, 
we can only hope for success if we trace out step by 
step the historic course of its origin and of its growth. 
Everything is, in a word, known only by the history of 
its evolution. This fundamental law holds of human 
knowledge as of all other organic functions. It will, 
therefore, clearly be essential to glance rapidly at the 
stages of development through which zoology has 
passed in the course of the civilised life of man. 



These stages of development are, in truth, sufficiently 
strange, and in many respects are unique. For if we 
include under the conception of zoology, as is natural, 
the complete knowledge of animal life in all its various 
forms and functions, the morphology and physiology 
of animals as a whole, we are encountered at the out- 
set by the wonderful fact, that the different branches 
of animal study have developed isolated and uncon- 
nected one with another in a very remarkable fashion ; 
whilst, on the other hand, they have been on occasion 
in very close connexion with various other sciences. 
Thus, the major part of our knowledge of the anatomy 
and physiology of animals has arisen from the wants of 
human anatomy and physiology, and these, on their 
part, have been cultivated in the main on behalf of 
medicine. This also holds of a part of development, 
viz., that of the individual or embryology, whilst the 
other chief division of that subject, the palaeontological 
development of animal species and phyla, stands 
altogether apart from the former, and is the servant of 
geology. Psychology, an integral part of physiology, 
was wholly separated from it, and placed under the 
guardianship of a purely speculative philosophy that 
knew nothing of the zoological basis essential to all 
psychology. Finally, a system of animal classification 
altogether distinct from all these studies, and having 
only to do with the description and grouping of the 
different species of animals, appeared. Although this 
systematic zoology ignored for the most part the 
branches of knowledge just enumerated, and chiefly 
borrowed details from anatomy alone, it claimed none 
the less to be the " true " zoology. This claim might 
seem well-established if the amount of zoological litera- 
ture, and the tables of contents of the handbooks of that 
science were regarded as the means of estimation. The 
literature and books have, in fact, been devoted in the 
main to systematic zoology alone. Of late years, 
physiology on the one hand, anatomy on the other, have 
been at warfare on this point, and even now each 
science is by some regarded as the " true " zoology. So 
far is this contest from its termination, that even up to 
the present time among our recognised leaders in 



science, ideas as to the contents and extent of these 
two sciences differ widely, and now this part, now that, 
is put forward as the veritable zoology in opposition to 
all others. 

To the unprejudiced observer, standing" without the 
limits of the contest, this must appear the more strange 
in that already Aristotle, the mighty naturalist of old, 
whom a grateful aftertime honors as the Father of 
Natural History, looked on the study of animals, rightly 
enough, as the totality of knowledge in respect to 
animals. His classic " History of Animals," in addition 
to the smaller writings on special details, his compara- 
tive anatomy work on the parts of animals, and his 
ontogenetic work on their reproduction and develop- 
ment give us a conception of the animal world so uni- 
versal, so large, that it is not difficult to conceive why 
this work has during more than fifteen hundred years 
enjoyed, as a textbook of zoology, an authority altogether 
without parallel. 

Until the sixteenth century no inquirer arose who 
would undertake to continue the vast work that Aris- 
totle had begun, or even to follow out in detail any 
special parts of the plan of knowledge that he had 
sketched. Men were rather contented with transcrib- 
ing, translating, and annotating the works of Aristotle. 

When on the discovery of the new world, and on the 
discovery of the sea passage to the East Indies, and of 
many other new routes in the fifteenth and sixteenth 
centuries, a crowd of new animals and plants, hitherto 
unknown, were brought to Europe — then Natural His- 
tory began to awake from its long sleep. The need of 
distinguishing, grouping, and naming the new forms 
acted first as a stimulant. This need was the more 
pressing the greater the number of different plant- 
species accumulated in the herbariums, or of animal 
species in the zoological collections. But at the begin- 
ning of the eighteenth century came the great reformer 
of Natural History, who with boldness and strength 
grasped the gigantic and increasing mass of materials, 
arranged them with a master hand, and for the first 
time introduced into our artificial classifications a 
rigidly logical system. In 1735 appeared the epoch- 


making work of Carl Linne (Linnaeus), and with it the 
firm foundation for all after systems of classification of 
animals and plants was laid. The binary nomenclature 
of Linnaeus that was fully worked out in this book, the 
double method of naming of organic forms, based on 
the distinction between the species and the genus, 
proved so practical that down to the present time it is 
in general use. 

Now, it was for the first time possible to arrange the 
whole mass of animal and plant forms, and to place 
them in the artificial plan of this system under the 
particular names of genera and species. Very shortly, 
as a consequence of this, whole armies of naturalists 
turned to the newly-discovered domain of the classifi- 
cation of organic beings. On the one hand the dis- 
tinction between and classification of the many different 
species of animals and plants, on the other the aesthetic 
joy in beauty, or even the newborn interest in the 
strangeness of external form, exercised such a power of 
attraction that the great majority of naturalists after 
Linnaeus found in this work alone complete satisfaction. 
Even at the present time, when anatomical and physio- 
logical work as opposed to classification has become 
so strongly developed, the literary ability, and, at all 
events, the numerical importance of the professors of 
the latter are so marked that they are still amongst a large 
number of people regarded as the " true " zoologists. 
Even to-day more scientific men concern themselves 
with the collection, preservation, classifying, naming 
of animal and vegetable forms than with their anatomi- 
cal and physiological investigation, or with their em- 
bryology. Even at the present day these constitute by 
far the larger part of zoological and botanical literature. 

This imposing past, and the powerful position of 
classification, compel us to state at once our own 
opinion of this method, of study, and the various and 
widely diverse ideas held as to its value and signifi- 
cance. For whilst some, following Linnaeus, see in the 
systematic arrangement of organic things the real aim 
of natural history, and othe s see in such an arrange- 
ment only an artificially arranged expression of the 
sum of our biological knowledge in the manner of the 


■collector of stones, others deny that classification has 
any scientific value at all. 

In order to arrive at a right judgment in this strife 
of opinions, we must distinguish between those purely 
artificial systems of a large number of people whose 
ideal is the most perfect possible zoological museum 
and herbarium, and the systems of those who see in the 
natural system of organisms the hypothetical expres- 
sion of their actual descent, in the approximate deter- 
mination of which they are pursuing a scientific end 
as lofty as it is difficult. 

Classification of the first order, the museum zoology 
and herbarium botany, as they have hitherto been 
carried on to an altogether excessive extent, hardly 
deserves the name of a science. For science must, 
as science, be able to show a definite store of general 
results and laws ; it must strive after the understanding 
of phsenomena and the knowledge of their causes ; it 
must never concern itself with the mere knowledge of 
isolated facts. But this last is in pure classification the 
sole end and aim. Classification desires nothing further 
than to know all the separate forms of animals and 
plants, to describe them, to distinguish them by name. 
But a merely descriptive natural history of this kind 
can never be a science. For the conception of a merely 
descriptive science is a contradiction in terms, a con- 
tradictio in adjecto. We are far from under-estimating 
the great practical work of descriptive classification. 
It is indispensable, both for zoological and botanical 
collections, and for the special scientific study of ani- 
mals and plants. It is as indispensable as these very 
collections, and the whole use of zoological and botani- 
cal knowledge in practical life depends upon it. But 
a science that is put into practice is no longer a pure 
science. It is an art, and we must consider the purely 
descriptive classification of organic forms as much an 
art as are medicine, pharmacy and farming, to all of 
which it serves also in an especial degree as hand- 

That true scientific classification which sees and 
searches out in the natural system of organic species 
their actual genealogy is altogether different from 


artificial, descriptive systematising. This genealogical 
treatment and conception of the natural system has 
only become possible in recent times, since Charles 
Darwin, by his reform of the theory of descent, led us 
to a true, causal understanding of the phenomena of 
the organic world. Without doubt, a long time will 
elapse ere the last branch of our systematic genealogical 
tree will be perfected, and the end and aim of our 
genealogical classification be fully attained. But the 
future belongs to it only by that genealogical concep- 
tion of the natural system, which sees in the categories 
or divisions of that system, in the classes, orders, genera, 
species, nothing but divergent branches of one veritable 
ancestral tree, which recognises in the relationship of 
form in organisms their blood-relationships ; only by 
this genealogical conception of the system of forms 
will classification rise to the height of a true science. 

Further, descriptive classification has, during the last 
century, been compelled to approximate more and more 
to the true natural system, as it has been compelled 
more and more to make the collective relationships of 
structure and development in organic forms the broad 
basis of their systematic differentiation. The earlier 
classification of Linnaeus was purely artificial in that it 
made use of single, and by preference external, easily 
recognised marks for the distinction of species, genera, 
and even of the larger groups, orders and classes, and 
dealt with these marks, or at least tried to deal with 
them rigidly, as one would with a monetary system. 
More recent classification, especially since the beginning 
of this century, in lieu of these, rather kept in view the 
nature of the structure as a whole, and especially the 
important internal relationships of parts ; and during 
the last ten years has based itself essentially upon 
embryology. Whilst, at the present time, this last, and 
especially the history of Evolution as a whole, are more 
and more understood in their essential worth, classifi- 
cation unconsciously takes its direction more and more 
decidedly from the genealogical and truly natural 
system, and in the very doing so of necessity loses, 
in many instances, its logical character. For a rigidly 
logical classification must necessarily be often artificial, 


and for many reasons cannot be reconciled with a 
genealogical, natural classification. 

The synthetic, genealogical classification of the future 
will therefore aid more than all its predecessors in 
gathering together the various isolated branches of 
zoology into one natural centre, the true natural his- 
tory, unifying them in one all-embracing historic science 
of animal life. The analytic, descriptive classification 
of the past did exactly the opposite of this, whilst it 
was ever striving to press itself forward as the " true " 
zoology, and to shut out from the domain of the so- 
called " true " zoology those branches of science which 
in reality gave it such intrinsic worth as it had, ana- 
tomy and embryology. This strange idea explains 
that isolation in relation to other sciences already men- 
tioned, in which anatomy and the other branches of 
zoology are in the main evolved. 

That part of scientific zoology that more than all 
others ought to have been cared for by classification, 
morphology, or anatomy and embryology, has, in fact, 
up to the beginning of our century worked in absolute 
independence of the widely prevalent systematic 
zoology. And even now-a-days we find the question 
asked by scientists of position, and in handbooks that 
have many readers, whether, in point of fact, the com- 
parative anatomy of animals belongs to zoology or not. 

Of course, Aristotle, long ere the present time, recog- 
nised that the natural history of animals involved the 
knowledge of their internal structure, and he had him- 
self dissected many subjects. Nay, his great predeces- 
sor, Democritus of Abdera, founder of the atomic theory, 
had carried his zeal for anatomical investigation so far 
that his fellow-citizens thought him mad and banished 
him. But in the following age the knowledge of the 
internal anatomy of animals was especially forwarded 
through medicine, which had, ere this, found the abso- 
lute necessity of acquiring a thorough knowledge of the 
internal structure of the human body. But prejudice 
and superstition during all the ancient time and the 
middle ages, put the greatest hindrances in the way of 
the dissection of human bodies. Hence, men had to 
take refuge in the anatomy of Mammalia most nearly 



allied to man ; and from the internal structure of these 
drew their conclusions as to what ought to be in man. 
The Roman physician, Claudius Galenus, who lived in 
the second century after Christ, and whose writings on 
human anatomy and pathology enjoyed boundless au- 
thority up to the fifteenth century, drew his knowledge 
of human anatomy in the main from the dissection 
of apes. Even in the fourteenth and fifteenth centuries 
men only dared carry on the study of human anatomy 
in secret places ; especially after Pope Boniface VIII. 
had fulminated the mighty curse of the church against 
all who dared to dissect human corpses. Hence the 
physicians, athirst for knowledge, were limited to the 
anatomy of the dog, the horse, and the allied domestic 

In this manner much knowledge, as to the internal 
structure of the body of the higher animals, was 
acquired. But, for the first time, in the eighteenth 
century men began to investigate, and to compare the 
anatomy of the lower animals. Towards the end of 
that century Pallas, Poli, and Camper, especially had 
prepared the ground on which, at the commencement 
of the present century, Cuvier, for the first time, was 
able to erect, as an independent edifice, the study of 
comparative anatomy. 

Amid the many and inestimable services rendered 
by Cuvier to the advancement of zoology, stands out 
pre-eminently his marking-off the great natural groups 
that he called branches or types of the animal king- 
dom, as characterised by certain essential, constant 
fundamental points in their internal anatomical struc- 
ture. By this method, the most important general 
results of comparative anatomy were at once, and for 
the first time, made of value in the art of classifying 
animals, and the foundation of a natural system laid. 
As Cuvier had at once a wide knowledge of animal 
classification, and a thorough acquaintance with com- 
parative anatomy, the internal connexion between these 
two studies must have been quite clear to him, so that 
he was able to speak of comparative anatomy as at once 
the forerunner and as the goal of zoology. 

Nevertheless, this fusion of the sciences was far 


from being generally recognised. In fact, for a time, 
there appeared to be a more acute separation made of 
one from the other, when, on the one hand, men 
turned their attention to the inquiry into the internal 
structure, only possible in the higher animals by dis- 
section, i.e., comparative anatomy, and on the other to 
the description of external forms, that is true or sys- 
tematic zoology. But in this there was a double 
blunder. For, in the first place, the mere anatomical 
dissection of animals, and the description of their in- 
ternal structure, is not comparative anatomy, but is 
rather mere zootomy. But zootomy deals simply with 
analysis and description, whilst comparative anatomy, 
on the other hand, as its name implies, working syn- 
thetically and comparatively, rises to the rank of a 
true philosophic science, a rank to which the other 
never can lay claim. Zootomy remains a pure art, like 
human anatomy, so long as the latter does not work by 
the method of comparison and synthesis. 

But, in the second place, it is also a blunder to com- 
prehend under anatomy, only the knowledge of the 
internal structure, and not that of external form. 
Anatomy is rather the sum-total of the knowledge of the 
fully-developed, or perfect forms of organisms, whether 
these appear externally on the surface of the body or 
not. When, e.g., Savigny, in the innumerable and 
manifold parts of the mouth of insects, recognised one 
and the same fundamental form, a true so-called type, 
this was pure comparative anatomy, although the parts 
of the mouth of insects are altogether external and 
are, moreover, of constant service in systematic zoo- 
logy, although this last use is, of course, only in the 
opposite, analytical, zootomical sense. 

Like the study of organs, which forms the chief part 
of comparative anatomy, the study of their elemental 
parts, the study of tissues or histology has also, under 
the stimulus of medicine, taken its origin from human 
anatomy. It is true that the great Italian, Marcello 
Malpighi, more than two hundred years ago, began, by 
aid of the microscope that had just been discovered, to 
examine the minuter structure of the animal and of 
the plant body, and the composition of the different 



tissues. But neither Malpighi and Leeuwenhoeck, nor 
the microscopists of the eighteenth century, were able 
to do more than make a heterogeneous collection of 
disconnected facts. Even after Xavier Bichat, 1801, 
had given, in his " Anatomie Generale," the first con- 
nected account of human histology, nearly forty years 
passed ere, led by Schleiden's vegetable-cell theory, 
enunciated a little while before, Theodor Schwann 
published his epoch-making, " Enquiries into the Cor- 
relations in Structure and Growth between Animals and 
Plants." In this it was shown that the animal body, like 
that of the plant, was composed of independent ele- 
mentary organisms, or individuals of the first rank, of 
cells, and that every multicellular organism arose from 
a simple cell. This cell-theory, however, did not do 
its remarkable work nearly so thoroughly or so quickly 
in zoology as in botany, where, in a very short space 
of time, the study of cells became so completely the 
main part of anatomy, that the two ideas were often 
regarded as identical. But the study of human cells 
and its companion, the histology of the vertebrate 
body, soon received an exceedingly powerful impulse, 
as soon as scientific medicine rightly understood its 
fundamental significance. The acute Virchow espe- 
cially, by means of his cellular pathology, was able to 
grasp and to demonstrate the inner nature of the cell 
life more completely than the vast array of histolo- 
gists clinging only to the study of external cell-forms. 
The histology of the invertebrate animals, however, 
was very behindhand, and the last century only has 
begun, in comprehensive fashion, the unveiling of the 
immense treasures that lie hidden there. It is at all 
events the greater pity that to-day even the real mean- 
ing of cell-life is altogether lost by the majority of the 
zoologists in name, and that histology is, to a far 
greater extent than morphology, regarded as a study 
about which the true zoologist has no need to trouble 

The study of the development of animals has 
grown up yet more widely sundered from systematic 
zoology than comparative anatomy and histology. 
This holds good of both its branches, of the develop- 


ment of the individual animal, commonly called em- 
bryology, more accurately ontogeny, and of that of 
animal species and phyla, palaeontological development 
or phylogeny. 

The natural history of man, and the interest taken 
therein by scientific medicine, gave rise to the former. 
Human anatomists were obliged, of necessity, to con- 
sider the structure and development of the human 
embryo. But as the study of embryonic development 
in its earliest stages in man and in other Mammalia is 
a difficult pursuit, men turned at first to those nearly 
allied Vertebrata, the birds, in whom the development 
of the egg can with ease be traced from the commence- 
ment. Despite the fact, that even in the seventeenth 
century a number of facts as to vertebrate embryos had 
been given as result of ancient and of more recent in- 
vestigation, Caspar Fredrich Wolff was the first to prove 
in his "Theoria Generationis " (1759), that the true 
nature of animal development was a veritable Epi- 
genesis. Even then half a century elapsed ere this idea 
received the recognition it deserved. 

When, at the beginning of the present century, 
embryology received a fresh and powerful impulse, 
especially at the hands of Pander and Baer, again the 
Vertebrata, and chiefly the Mammalia and Aves, were 
the animals about whose development man, having an 
eye to that of his own species, concerned himself in the 
main. It is true that the far-seeing Baer, in his " Develop- 
ment of Animals," which dealt principally with the 
Vertebrata, pointed out in broad outlines the chief 
characters that distinguished the main groups of the 
invertebrate animals in their ontogeny. But some ten 
years later, more detailed and comprehensive studies 
of the history of development of the different Inverte- 
brata began to be made. Even at the present time, 
despite the many brilliant discoveries of the past ten 
years, our knowledge of the development of the Inverte- 
brata, as a whole, is far behind our knowledge of the 
Vertebrata. But this much at least is gained, that 
every day in zoology and botany, the real scientific 
student is learning to recognise development as the 
essential fundamental through which a true understand- 


ing of the anatomy of the adult form can alone be 

It is true, that hitherto the recognition of this fact 
has been limited to the one branch of development 
just named, that of the individual animal. The second 
division of the subject, of no less significance, has on 
the contrary been, until quite recently, neglected in 
the strangest fashion ; that is, the palaeontological 
development of animal species, phylogeny. This has 
to investigate the changes of form that the few princi- 
pal classes of the animal kingdom, the phyla or stems, 
have passed through during the long periods of the 
earth's history in the incessant transformation of species 
that has taken place. 

When first Charles Darwin, in 1859, published his 
epoch-making theory of Natural Selection, and gave, 
in the doing thus, to Lamarck's theory of descent, enun- 
ciated fifty years earlier, its impregnable causal founda- 
tion, it became possible for men to deal in earnest with 
this momentous, this interesting branch of zoology that 
had until then not existed even in name. It became 
clear that the empirical materials of this history of 
ancestral forms had been accumulated in a very differ- 
ent domain of science, without any apparent connexion 
with zoology. For the fossil remains of animals that 
lie buried in the womb of earth, and as "medals struck 
off at the creation," tell us the history of animals dead 
thousands of years ago, were studied at first, and chiefly, 
on account of their bearing on the history of the de- 
velopment of the globe. The geologists were the first 
to give close attention to petrefactions, and hence 
palaeontology has made its way hitherto wholly in aid 
of geology. 

Now, the value of fossils to the geologist lies chiefly 
in the fact that they tell him the relative ages of the 
strata that have been deposited from the water, and are 
now lying one upon the other. The zoologist, on the 
other hand, recognises in petrefactions the remains of 
dead and gone forefathers and relations of the animals 
now living. He has to aim at building up from the 
regular historic succession in which these occur, a correct 
ancestral history, the history of the ceaseless transfor- 


mation of species. Hence, e.g., is it that the different 
mammalian fossils have the deepest interest for zoolo- 
gists, the least for geologists. On the other hand, the 
many fossil species of Gasteropoda and Lamellibranchi- 
ata that have for the geologist the highest significance, 
as guides in the study of the formation of mountains, 
are of only secondary importance in the phylogeny of 

No mistake in the treatment of zoology has, up to 
the present time, led to such evil consequences as this 
unnatural separation of the two branches of the study 
of development. It was impossible for man to under- 
stand the essential nature of organic evolution so long 
as ontogeny and phylogeny, the development of the 
individual and that of the species, had no concern one 
with the other. For, in truth, these two halves of the 
science of development stand in the most intimate 
causal connexion. The series of forms that the organic 
individual runs through, in its short, swift development 
from the egg, repeats for us in broad, general outlines 
the series of forms that its ancestors have run through 
since the beginning of organic existence in the long, 
slow course of their ancestral history, or transformation 
of species. Or, in other words, the history of the in- 
dividual, or ontogeny, is a brief, rapid repetition, under 
laws of heredity and adaptation, of the history of the 
race, or phylogeny. 

The clear recognition of this most important relation 
is of greatest moment, not alone for the estimation of 
the science of development, but for the whole of 
zoology. But the fact that this was first clearly under- 
stood but recently will enable us to grasp how very 
backward even now is our science. The natural 
genealogical classification which regards the natural 
system of organic species as their genealogical tree can, 
as we have seen, only develop unfettered as a conse- 
quence of this recognition. 

The branches of zoology that have been mentioned 
so far, anatomy and classification, development of the 
individual and development of the race, all belong to 
that wide domain of our science that is named the 
study of forms, or morphology. Side by side with 


this, as a second part of zoology, stands physiology, the 
study of the life-phsenomena of animals. As morpho- 
logy is divided into the two chief branches of anatomy 
and development, so physiology is divided into the 
physiology of labor and the physiology of relation. 
The former investigates the intrinsic functions of the 
organism, the latter its life-relations to the outer world. 
Further, these two orders of study have taken origin 
from departments of natural science quite different 
and widely separated. 

That which has to do with the environment, the 
physiology of relation, or the study of the relationship 
between the animal organism and the outer world, is 
again divided into two parts, oecology and chorology. 
By oecology we mean the study of the oeconomy, the 
house-keeping of animal organisms. This has to do 
with the totality of the relations of the animal, both to 
its inorganic and its organic surroundings, and, above 
all, the amicable and inimical relations to those animals 
and plants with which it comes into direct or indirect 
contact : in a word, all those complex mutual relation- 
ships that Darwin has shown are the conditions of the 
struggle for existence. This oecology (often incorrectly 
called biology in its limited sense) constituted, until 
the present time, the chief part of so-called " natural 
history " in the ordinary sense of the word. It grew, 
as the many popular natural histories of former and of 
recent times show, in very close connexion with 
ordinary classification. Uncritically as this economy 
of animals was treated on the whole, it at all events 
had the merit of keeping alive an interest in zoology 
in many minds. 

Much less interest was for a long time taken in the 
other branch of the physiology of relation, viz., choro- 
logy, i.e., the study of the geographical and topogra- 
phical distribution, of the horizontal and vertical 
limits of animal species, or the geography of animals in 
the widest sense of the word. Until the present time 
this was a waste chaos of different facts, heaped 
together and not understood, in which even an Alex- 
ander Humboldt and a Carl Ritter could only now and 
again create any deep interest. Of late, thanks to 


Darwin's new fundamental idea, the theory of descent, 
it has become possible to understand the geographical 
and topographical distribution of organic species in 
relation to their mechanical causes, and to explain them 
as in their real nature active natural processes, essen- 
tially conditioned by the migrations of varieties and 
their transformation in the struggle for existence. 
Although, therefore, only its first rudiments are grasped, 
yet chorology, like oecology, gives us a glimpse into the 
mass of interesting results that the future will bring 

Another main division of physiology that we noted 
previously as opposed to external physiology, or that of 
relation, was internal physiology, or that of conservation. 
This investigates the life of the organism in relation to 
itself, the functions of its organs, and especially those 
very important and very general life-phsenomena, the 
functions of self-preservation, of growth, of nutrition,, 
and reproduction. This second main division of physio- 
logy has, like anatomy, taken origin, quite independ- 
ently of the first, from medicine. As soon as scientific 
medicine recognised that in order to a correct know- 
ledge of the human body in disease, not only the know- 
ledge of its organisation, but also of all its life-phaeno- 
mena was an indispensable preliminary, it was forced 
to make human physiology the forerunner of pathology. 
But as the human organism was not available for many 
physiological inquiries, especially for the investigations 
and experiments connected with vivisection, the human 
physiologists very early turned to the Vertebrata most 
nearly allied to man, amongst which the faithful dog 
and the luckless frog have chiefly furnished the unfor- 
tunate subject-matter for experimental physiology. Of 
course this inquiry into certain life-phenomena in in- 
dividual Vertebrata that arose from practical needs, was 
far from leading to a real comparative physiology. 
This even now exists only in conception and in 
outline, and the one-sidedness of the physiologists is 
not less blamable in this respect than the indifference 
of the systematic zoologists. This much, however, is 
already gained, that the metaphysical spectre of a so- 
called " vital force" is once and for ever banished, not 


only from the domain of human, but also from that of 
all animal physiology. In a true scientific investiga- 
tion and explanation of lif e-phsenomena, there can now 
be no more talk of this mystic product of the confusion 
of dualism which has done so much harm and made so 
much perplexity — now as an active life-principle, now 
as a final cause working towards a designed end, now 
as an organic creative force. We know to-day that all 
the life-phenomena of the lower animals, as of man, 
follow, under an absolute necessity, great mechanical 
natural laws ; that they are brought about by no final 
causes, but by mechanical or efficient causes ; that they 
are based in the last analysis upon physical and chemi- 
cal processes, on, in fine, the delicate and complex 
motor-phaenomena of the very small particles that make 
up living bodies. Here also in physiology, as in mor- 
phology, full light has been thrown, of late, on the 
natural and mechanical connexion of all phenomena 
by the descent-theory of Lamarck and Darwin. This 
shows us how, like the forms of cells and organs, their 
special life-movements also, their specific functions, 
have gradually, step by step, evolved along the lengthy, 
the toilsome, path of ever-advancing development and 

In no department of zoology will this knowledge 
bring about greater revolution than in that of animal 
psychology, to which we must, of necessity, in conclu- 
sion, give special attention for a moment. For, without 
doubt, the study of the animal soul has developed in 
more complete isolation, and is therefore more behind- 
hand, than all other branches of zoology. Even human 
psychology, whence all the comparative psychology of 
the lower animals has ever sprung, has, as yet, labored 
wholly in the service of a speculative philosophy, that 
from the outset up to the present time has despised the 
indispensable fundamentals of empirical physiology. 

What should we say to-day of a botanist who wished 
to separate the soul-life of plants from their other life- 
phaenomena, and to relegate the study of the latter to 
empirical physiology, the study of the former to specu- 
lative philosophy ? And the soul-phsenomena of many 
plants (as e.g. the sensitive Mimosa, the Venus' Flytrap 



and even our native berberry flowers) present a higher 
degree of perfection than those of many lower animals, 
as e.g. sponges, many corals and ascidians. But these 
last, the ascidians, among all the invertebrate animals, 
have the closest blood-relationship to the Vertebrata ; 
and amongst them we find such an unbroken continuity 
in the graduated evolution of the soul-life that we can 
make out a connected, progressive series up to and 
through many Amphibia, whose spiritual development 
is far inferior to that of higher Vertebrata, and thence 
to many Mammalia that, in all probability, reach in 
mental development beyond the lowest conditions in 

When once we, turning from this dark domain, ever 
more and more darkened by mystic speculation, follow 
those methods of inquiry that are our best guides in 
biology to the true end, the two methods of comparison 
and the study of development, we must, perforce, be 
led to the conclusion that the human soul-life, like 
other vital functions, has slowly evolved during man's 
history in the struggle for existence, step by step with 
the advancing perfection of the nervous system. Con- 
sequently, inquiry into this subject can fall to the lot of 
no other science than comparative physiology as a 
branch of zoology. 

This is before all the point at which zoology comes 
into closest contact with speculative philosophy. But 
our care must be so to work that this contact may lead 
not to a hostile repulsion, but to a closer approximation. 
For zoology, to my thinking, can dispense with specu- 
lative philosophy as little as any other natural science. 
It is as little able to lead to lasting results without 
speculative philosophy, as the latter without the em- 
pirical basis of natural science. The highest aim and 
problem of sound natural science is the general and 
philosophical knowledge of nature. The deepest fun- 
damentals and the very pillars of sound philosophy are 
the physiological laws of empirical origin. Only by 
the most complete mutual interpenetration and co- 
working can empirical natural science and speculative 
philosophy attain the end they have in common, the 
knowledge of the truths of nature. 


Those scientists who, proud of their absolute empiri- 
cism, think they can advance science without philoso- 
phical thinking are guilty of the terrible confusion of 
ideas, and of judgment, and of the astounding blunders 
in natural logic, which we meet with generally in 
zoological and botanical literature, confusion and 
blunders which must draw from every philosopher a 
gesture of pity and of regret. The philosophers, on 
the other hand, who think they can arrive at the 
knowledge of general laws by pure speculation, without 
any empirical and scientific basis, are building castles 
in the air, that the first good empiric, with the help of 
actual experiments, can blow into the infinite. 

Nothing demonstrates more clearly the necessity of 
the most complete mutual interworking between ana- 
lytic empiricism and synthetic philosophy, in order to 
a true advance of science, and especially of zoology,, 
than the great question which at present is engaging 
the thoughtful in all parts of the earth, the question as. 
to man's place in nature. Even this question, how- 
ever, we regard as already decided in the sense of the 
theory of descent. In conformity with that theory we 
admit a graduated evolution of the human race from a 
series of lower vertebrate forms. We base our idea upon 
the consensus of opinion of the greatest scientific men 
now living. Of these we will name only the great 
Englishmen, Darwin, Lyell, Huxley, Hooker, Spencer, 
Lewes. In their name we bid the German men of 
science nearer home be silent. 

In opposition to those able and thoughtful men who,, 
ranking among the many foes of this theory, are of an 
opinion opposed to mine, I cannot but lay especial 
stress on the fact that this question of questions is in 
the strictest sense of the word a purely zoological one, 
and that the battle-field on which it must be decided 
is the domain of scientific zoology alone, i.e., the 
empirical and philosophical study of animals. For the 
zoologist alone, who has certain morphological and 
physiological knowledge, and who knows how to make 
use thoughtfully thereof in Catholic fashion, can 
rightly estimate the immense value of the proofs that 
the theory of descent in its application to man has been 


already established beyond all possibility of contra- 
diction. When, therefore, speculative philosophers, 
without the indispensable knowledge of anatomy, 
-embryology, and physiology, wish to deal with this 
question, their contributions to its solution are as 
worthless as those of the rude empirics who, from 
want of philosophic grasp, are unable to combine and 
estimate the exact value of a series of facts. Although 
now, unfortunately, the majority of the many treatises 
that aim at deciding once for all man's place in nature 
belong to one or to the other of these two categories, 
still on the other hand its definite determination is, by 
the efforts of true empirical and philosophical zoology, 
advanced so far that in a short time the prophecy of 
Lyell ought to come true : "It will come to pass in this 
question as ever when a new and surprising scientific 
truth is discovered, that men will say first, it is not 
true ; second, it is antagonistic to religion ; lastly, it 
has been known these many years." 

As I close my explanation of the ends and the signi- 
ficance of scientific zoology with this hint as to its 
highest problem, I dare to hope that I have given an 
approximate idea of the extraordinary capabilities of 
development, the momentous future of our young 
science. Whilst the study of animals existed for a 
century and a half as an entirely isolated science, 
whilst it has lived the greater part of this time in child- 
like unconsciousness, ignorant of the forces slumbering 
within it, and without a dream of its own lofty aims, 
it has since the commencement of our century begun 
to gird itself up for higher stages of development, and 
to gather to itself its own integral parts that have been 
evolving unconnected with each other in the service of 
other, stranger sciences. Since, ten years ago, Charles 
Darwin fashioned that unifying bond which unites all 
these orders of thought, once so widely asunder, into 
one majestic whole, since in doing thus he breathed 
into the giant form of zoology, born again, a new, a 
forceful life, the range of view, the ends and aims of 
our science have grown beyond measure. From all 
sides it is drawing to itself keen workers, athirst for 
knowledge. In all directions there is promise of the 


richest harvests. And if we think but little of all other 
acquisitions of zoology, its indissoluble connexion with 
empirical and philosophical anthropology alone would 
give it a meaning of the very deepest. The monistic 
philosophy of the future will be unable 10 dispense 
with the comparative study of animals on this one 
ground alone. From the small, despised seed grain of 
zoology a tree of knowledge shall evolve, that will in 
the coming years gather all other sciences beneath its 
shade, and from whose roots all shall draw something at 
least of sustenance. 

Date Due 

4 1970 „ 

'EAC?£ A 

3 Poq 1Q7R 

(jjjjjf CAT. NO. 23 233 PRINTED IN U.S.A. 


S'2 ■ 

1. Tectology, or study of structure. 
(Histology, Organology, Blastology, 

2. Promorphology, or study of types. 
(Geometric ideals, types and actual 

c £ 


3. Ontogeny. History of embryo. 

(Embryology, metamorphosis, life- 

4. Phylogeny. History of race. 

(Palaeontology, genealogy, classifica- 


O 5i 
r-3 ^ 

S3 <~ 

> .2 
t— < v - 

5. Physiology of vegetative functions. 
(Digestion, nutrition, circulation, 
respiration, reproduction.) 

6. Physiology of animal functions. 

(Sensation, motion, will, imagination, 

7. CEcology. Study of the household. 
((Economy, habitat, relation to other 
organisms, parasites.) 

Chorology. Study of distribution. 
(Geography and topography of ani- 
mals — their diffusion.) 




FOR the last ten years a philosophic movement in 
natural science has gone on with steadily increas- 
ing force, whose waves have reached ever wider and 
wider circles, and have produced in the realm of philo- 
sophy a corresponding movement of a scientific order. 
With the increase in the number of new discoveries 
that the unwearied energy of many observers is making 
in all parts of the domain of natural science, the more 
powerfully are all thoughtful scientific men feeling 
the necessity of a unified philosophic standpoint for 
their understanding of these discoveries, of rising from 
the knowledge of facts to that of causes. On the other 
hand, the less the ability of those many systems of 
metaphysical speculation that are in deadly hostility to 
empiricism to hold together the remnants of their 
quondam following, the more is the conviction forced 
on the far-seeing philosophers, that on the secure 
basis of experimentally acquired facts can an enduring 
system of knowledge be built, and that hence the 
knowledge of facts must precede the knowledge of 
their causes. 

Of all the many things that this remarkable approxi- 
mation of philosophy and natural science has led to 
and has favored, without doubt the most important is 
the origination of the study of development, to which 
Charles Darwin gave its first impulse by his work on 
the " Origin of Species." Although this great scientific 
observer, with his usual care, abstained from giving to 
his theory of Natural Selection and the theory of de- 
scent as modified by the former the name of a philoso- 
phic system, and from drawing thence all the conse- 



quences interwoven with his theory ; yet no careful 
observer can any longer doubt that the vastest conse- 
quences of Darwin's writings lie, not in the immense 
wealth of the experimental facts he has gathered to- 
gether, but in the intelligent explanation and co-ordi- 
nation of those facts by means of the common bond 
of the theory of Evolution. But this unifying explana- 
tion of the different orders of phsenomena is a philo- 
sophical fact. 

I tried ten years ago, in my " Generelle Morphologie 
der Organismen," to make systematically the first com- 
prehensive attempt to lay down the philosophical bases 
of thought for the new theory of Evolution, and 
especially to construct the mechanical foundations of 
the science of organic forms in the light of the theory 
of descent. Though this attempt failed and was in 
many respects premature, yet many statements that 
were then made for the first time have since 
proved in accordance with fact and productive of re- 
sults. In especial does this hold true in respect to my 
enunciation of the two main branches of the study of 
organic Evolution and of the causal connexion between 

As long as we understood by the study of develop- 
ment, after our blundering fashion, only that of indi- 
vidual organic forms, the so-called embryology and 
metamorphology (embryonic and post-embryonic de- 
velopment), both these were included under the term 
ontogeny, or the history of the germ. But this same 
ontogeny is but a main division of biogeny, or the 
comprehensive "history of development of organisms." 

As a second division, opposed, in a sense, to the 
former, we have the palaeontological development of 
species and phyla, the series of forms that have evolved 
in an unbroken succession of numberless generations, 
from the first appearance of organic life on our planet 
up to the present time. This history of the successions 
of generations, including palaeontology and genealogy, 
is best named in brief as ancestral history or phy- 

Ontogeny and phylogeny, the history of the indi- 
vidual and the history of the race, are, as I hold, two 



sciences standing in the closest and most immediate 
causal connexion. But these two subjects evolved to 
such different extents, that the older of them only, 
ontogeny, was at first held as the "true development ;" 
whilst the younger, phylogeny, only ten years ago 
became an independent branch of knowledge, and 
even at the present day is but little known — all this 
results, first, from the different experimental methods ; 
second from the dissimilar theoretical claims of the 
two studies. For the individual development of 
organisms, ontogeny, is a swift process of upbuilding 
that is run through in a very short time under our 
very eyes, whose external appearances we can follow 
directly from beginning to end, generally within a few 
weeks or months, rarely within a longer period of 
time. Step by step, stage by stage, we are able, by 
continuous observation, to trace out the changing series 
of forms through which every individual animal, every 
individual plant, passes from the ovum to the adult 
condition. On the other hand, the palseontological 
development of organisms, their ancestral evolution or 
phylogenesis, a slower process of upbuilding, occupies 
an enormous time, whose individual stages are to be 
measured by thousands of years, whose perceptible 
advances, corresponding with geological formations, 
must be measured by hundreds of thousands and by 
millions of years. The difference between a seconds 
clock, whose hand completes its circle in a minute, 
and a year clock, whose hand completes its circle in 
365 days, is not so great as the difference between the 
advance as with a breathless speed of embryonic 
history, and the movement hardly perceptible in its 
slowness of race history. But what weighs still more 
heavily on us is the deficient empirical basis of the 
latter. The palaeontological account of first creations, 
which ought to show us directly, in the successive 
series of fossils, the picture-galleries of the dead 
ancestors of the organisms of to-day, is, on well-known 
grounds, in the highest degree imperfect and deficient. 
Even as regards its very important fragmentary remains 
this branch of knowledge could scarcely be intelligible 
to us unless we were in possession of two others, of 


highest value as supplements and completions to palae- 
ontology — comparative anatomy and ontogeny. The 
lofty significance that in especial belongs to compara- 
tive anatomy in this connexion Carl Gegenbauer has 
shown, more than other men, in his excellent works. 
It is possible for us by a thorough knowledge, a 
thoughtful comparison, a critical use of these three 
most weighty branches of science, comparative anatomy, 
ontogeny, phylogeny, to understand the outlines of 
phylogeny, or the history of the race. 

The intimate causal nexus between ontogeny and 
phylogeny is therefore most important of all. This 
causal connexion, full of meaning, that the older 
thinkers had foretold for half a century, and on which 
Fritz Mtiller more than all others, except Darwin, has 
laid stress, may be formulated as follows. The series 
of forms through which the individual organism passes 
during its development from the egg to the complete adult 
condition is a brief, condensed repetition of the long 
series of forms that its animal ancestors, or the parent 
forms of its species, have passed through from the 
earliest ages of what is called organic creation to the 
present time. (Cf . my " Generelle Morphologic," vol. 
ii., pp. 295 — 300 ; " Zeitschrift fur Naturwissenschaft 
(Jena), vol., viii., p. 5 ; vol. ix., p. 409 ; vol. x. Supple- 
ment, p. 77.) 

The development of the embryo is an epitome of 
that of the race ; an epitome the more complete, the 
more the epitomised development, or palingenesis, is 
retained under heredity ; the less complete, the more 
the development that is not due to heredity, or ceno- 
genesis, is introduced by adaptation. 

That this biogenetic fundamental law is the true 
thread of Ariadne that will lead us through the involved 
labyrinth of ancestral history, I think I have demon- 
strated for the whole animal kingdom, by the example 
of the Gastrula in my Gastraea theory. In my monograph 
on the calcareous sponges, I have proved the law as 
regards the whole of the allied forms of this little group 
of animals, working it out in details in individuals. In 
my "Anthropogenie " I have tried to prove it by the 
special example of the history of the human embryo. 


Every advance in embryonic life is of either a palin- 
genetic or cenogenetic nature. 

As soon as heredity was proved to be the active cause 
of palingenesis, adaptation to be that of cenogenesis, and 
these two, working together, the essential factors of 
ontogenesis, the next aim appeared to be to work out 
more thoroughly heredity and adaptation as physiolo- 
gical functions of organisms. 

In my "Generelle Morphologie," I had brought 
heredity into direct physiological connexion with re- 
production, adaptation with nutrition, and thus I had 
shown the possibility of a mechanical conception, and 
a physical and chemical explanation of those two vastly 
important constructive functions of organisms. For if 
the physiology of to-day most righteously closes its 
doors against vitalism and teleology, if it rejects every 
mystical and supernatural action of the nature of a 
" life-force," and only allows within its domain physical 
and chemical, or in one word mechanical forces, it 
must seek the like mechanical explanation for the two 
most momentous of the life-functions that have to do 
with the upbuilding of the body, heredity and adapta- 
tion. And if our great critical philosopher, Immanuel 
Kant, rightly demands of natural science that it substi- 
tute at all points mechanical causes {ccmsce efficientes) 
for intelligent causes (causce finales) : if Kant, more- 
over, considers that mechanics alone provides a real 
explanation of phenomena, and that " in general there 
can be no natural science outside the principles of 
mechanics," we shall recognise this monistic basis as 
the only possible one for our history of development if 
it is to be a genuine natural science ; we shall seek 
mechanical causes, and mechanical causes alone, for the 
physical facts of organic Evolution. 

But modern physiology, to which alone this duty 
falls, has not, up to the present, ventured on the attempt 
really to grapple with heredity and adaptation in this 
sense, and to seek out the elemental changes involved 
in the two physiological functions. An attempt of this 
kind has as yet been made by Charles Darwin alone, 
when he enunciated, in 1868, his provisional hypo- 
thesis of pangenesis. This is to be found in the second 



volume of his valuable work on " The Variations of 
Animals and Plants under Domestication " (chap. 27). 
In the second edition of this work that has recently 
appeared (1875), Darwin has stated his hypothesis of 
pangenesis more fully, and with some modifications. 
I give now, in the first place, the gist of the theory in 
the words of its founder (vol. ii., p. 369) : — 

8 4 It is almost universally admitted that cells, or the units of 
the body, propagate themselves by self-division or prolifera- 
tion, retaining the same nature, and ultimately becoming con- 
verted into the various tissues and substances of the body. 
But besides this means of increase I assume that cells, before 
their conversion into completely passive or * formed material,' 
throw off minute granules or atoms, which circulate freely 
throughout the system, and when supplied with proper nutri- 
ment multiply by self-division, subsequently becoming deve- 
loped into cells like those from which they were derived. 
These granules, for the sake of distinctness, maybe called cell- 
gemmules, or, as the cellular theory is not fully established, 
simply gemmules. They are supposed to be transmitted from 
the parents to the offspring, and are generally developed in the 
generation which immediately succeeds, but are often trans- 
mitted in a dormant state during many generations and are 
then developed. Their development is supposed to depend on 
their union with other partially developed cells or gemmules 
which precede them in the regular course of growth. Why I 
use the term union will be seen when we discuss the direct 
action of pollen on the tissues of the mother-plant. Gemmules 
are supposed to be thrown off by every cell or unit, not only 
during the adult state, but during all the stages of develop- 
ment. Lastly, I assume that the gemmules in their dormant 
state, have a mutual affinity for each other, leading to their 
aggregation either into buds or into the sexual elements. 
Hence, speaking strictly, it is not the reproductive elements, 
nor the buds, which generate new organisms, but the cells 
themselves throughout the body." 

This is in brief the hypothesis of pangenesis of 
Charles Darwin. Its detailed explanation and the 
proofs in its favor, its application to the various phe- 
nomena of organic development, and especially its use 
in explaining the phenomena of heredity and adapta- 
tion, may be seen in the original work, a work which 
by its unwearied accumulation and critical investiga- 
tion of a mass of observations unequalled in extent, 
and by the grasp and clear presentment of those facts, 



shows us the great English naturalist " in his habit as 
he lives." 

Charles Darwin has himself from the outset called 
his hypothesis of pangenesis a provisional one, a first 
attempt at tracing back the totality of the processes of 
organic development to primary causes, at explaining 
them on one unifying causal basis. This pangenesis 
hypothesis has, like his Natural Selection theory, 
awakened the liveliest interest. It has met with as 
complete assent on the one hand as flat contradiction 
on the other. For my own part I have thus far not 
dealt with it in my works, and in my "Natiirliche 
Schopf ungs-geschichte " and " Anthropogenic," as in 
my other essays upon the study of Evolution, I have 
purposely as yet passed over the idea of pangenesis in 
silence. I am anxious to add that neither want of 
interest nor want of esteem for its keen-sensed author 
has led me to this silence. The real truth lies in this. 
Prom the outset, and now after I have been acquainted 
with pangenesis for eight years, I have found myself 
in complete mental antagonism to it, an antagonism 
the stronger and the more invincible the more I try to 
become acquainted with the hypothesis by continued 
reflexion on it, and try to grasp its usefulness in 
application to the various phaenomena of development. 
Only I was always, and am now, possessed of far too 
high a veneration of Charles Darwin, of far too sincere 
an admiration for his suggestive thought, to oppose a 
hypothesis so comprehensive and in large measure so 
well founded, or to attempt its refutation unless I were 
able to place something else in its stead. If to-day I 
venture on this attempt, I do so because certain germs 
laid down ten years ago in the "Generelle Morphologie," 
have in the interim developed into a definite hypo- 
thesis, that seems to me to have more intrinsic pro- 
bability of truth than pangenesis, a hypothesis of 
which I dare to hope that it may rise to the height of a 
genetic molecular theory. I call this hypothesis " the 
perigenesis of the plastidule," or making the attempt 
at a modern name that shall correspond as nearly as 
possible with the more classical one, " the wave-motions 
of living particles." 


To prevent misunderstanding, and to obviate the 
false view taken in connexion with my carbon theory 
and others of my theoretical speculations, that I wanted 
to introduce a new " dogma " into science, I state in 
advance that I also regard this perigenesis of the plas- 
tidule as only a provisional hypothesis, although I 
entertain the hope that the germ lies within it of a 
wide theory by which, it may be, in the future the 
whole of the phenomena of organic development will 
be explained in a rigidly mechanical fashion on physical 
and chemical elementary principles. At the same time 
I would explain, with reference to my esteemed friend 
and master, Charles Darwin, that my opposition is 
solely limited to his pangenesis. His other theoretical 
ideas, especially that most special work of his, the 
theory of Selection and all its consequences, I regard as 
perfect, and work for with all my powers. This ex- 
planation, is assuredly unnecessary in regard to Darwin 
himself. For the great English scientist, who intro- 
duced into biology a new epoch, fruitful beyond all 
calculation, to whom I myself owe my chief stimulus 
to work, is much too firmly convinced of my sincere 
gratitude and real sense of inferiority to him to be at 
all astonished at my combating pangenesis and pre- 
ferring perigenesis. This explanation, however, seems 
necessary on account of the tactics of many foes of the 
Descent theory, who hail any differences of opinion that 
appear in the camp of its adherents with delight as 
signs of its inaccuracy. Once again, therefore, I state 
explicitly that Darwin's theory of Selection and the 
new theory of Descent based upon it in my opinion are 
unshaken, and will not be in the slightest degree 
menaced by the speculative discussions that follow. 
Here we are simply concerned with a hypothesis as to 
the mechanical explanation of the most elementary 
conditions of Evolution. Whether pangenesis or peri- 
genesis is right, or even if both are inaccurate, the 
Descent theory of Lamarck and the Selection theory of 
Darwin are not affected in the least. 

For the foundation of our perigenesis, we turn to 
those ideas of the organic world that are based upon 
the nature of visible elemental parts, and that find 


widest expression in the magnificent cell-theory. Since 
the cell-theory was established for the plant-kingdom, 
by the genial botanist Schleiden, in Jena, in 1838, and was 
extended to the animal kingdom by Schwann in the 
following year, it has held its ground in botany as in 
zoology, in the morphology as in the physiology of 
organisms, as the firm basis and immovable starting- 
point for every elementary inquiry. Greatly as the con- 
ception of the cell has changed in the thirty-eight years 
that have elapsed since that time, grand as has been the 
building up within, the extension without, of the cell- 
theory, its fundamental conception has remained un- 
changed, and has ever risen to a higher value. This 
fundamental conception is, that we have to look upon 
the microscopic cells as independent living beings, as 
organisms that are physiologically and anatomically 
autonomous. Brucke has, therefore, called them very 
aptly, elementary organisms ; Virchow, life-armies ; 
Darwin, living units. Referring to the successive 
steps of the organic individuality (organ, person, stock), 
I have, in my " Generelle Morphologie," placed them 
as " individuals of the first order," at the foot of the 
anatomical study of individuality. Rudolf Virchow, 
more than all other scientific men, has the further merit 
of having extended the study of cells in this sense in 
all directions, and given, by his " cellular pathology," 
a firm histological basis to recent medicine. If I my- 
self have been able to contribute something to the 
earlier upbuilding of the study of development, I owe 
it in great measure to the ideas in cellular biology that 
the teaching of Virchow at Wurzburg gave me twenty 
years ago. In pursuance of his ideas, I looked upon 
every higher organism as an organised social unity, as 
a state whose citizens are the individual cells. As in 
every civilised state the individual citizens are inde- 
pendent to a certain extent, but at the same time are 
connected in the division of their labor one with 
another, and are subject to general laws; so in the body 
of every higher plant and of every higher animal the 
innumerable microscopic cells enjoy, to a certain extent, 
their individual independence, but are, as result of 
specialisation, differently constructed and dependent 



one on another ; in like manner, also, they are governed 
to a greater or less degree by the laws of the centralised 
whole. This political comparison, as complete as it is 
frequent in its application, is no strained picture of the 
imagination. It expresses a real fact. The cells are in 
reality citizens. The simile can be extended yet further, 
in that we may regard the extended and yet centralised 
animal body as a cell-monarchy, the plant-organism 
with its lesser centralisation as a cell-republic. Just as 
the different state-crafts, in the various governments 
existing at the present time among men, present us with 
a long series of advancing perfection from the rude 
hordes of savages to the most highly developed civilised 
states, so does the comparative anatomy of plants and 
animals reveal to us a long series of gradations of in- 
creasing perfection in the cell-states. At the lowest 
step in the association of cells and the formation of a 
community, we encounter the lower algae and fungi, 
the sponges and corals, who have in their limited 
specialisation and centralisation not risen above the 
level of the wild savage tribes. At the other extreme 
we find, high up in evolution, the mighty cell-republic 
of the tree, the wondrous cell-monarchy of the Verte- 
brata, in which the manifold improvement and speciali- 
sation *of the constituent cells have given rise to the 
origin of the most diverse organs, and in which the 
co-ordination and subordination of classes, the co-opera- 
tion for the welfare of the whole, the centralisation of 
government, in a word, organisation, have reached a mar- 
vellous height. As a rule, the blunder is made, that 
this great complex organism, with its judicious con- 
trivances, can only have been called into being as re- 
mit of a carefully thought-out plan of creation. Never- 
theless, this organised cell-state, with all its " plans," 
has evolved without any preconceived aim as neces- 
sarily from the co-operation and the co-ordination of 
its constituent cells in the course of time, as the civilised 
state of man in the course of a few thousand years 
evolves step by step as result of the transformations and 
advancing specialisation of its citizens. The history of 
human civilisation is the explanation of the history of 
the organisation of the multicellular organisms. m 


This political conception of the cell-theory, on 
which the whole understanding of biology hangs, is 
strengthened by embryology. Every organism, higher 
as well as lower, is developed primarily from a single 
cell, the ovum. As we are able to observe this uni- 
cellular origin for every individual, we ought, without 
a doubt, to assume it for every organic stem, for every 
group of related species. The unicellular embryo- 
form that has been verified by experiment is, according 
to our fundamental biogenetic law, the replica of a 
like, dead and gone, and now unknown ancestral form. 
The nature of such a unicellular ancestor is again 
exemplified to us in the clearest manner by the many 
unicellular organisms still living at the present day — 
the Amoebae, Flagellata, Diatoms, etc. These are 
savage hermits, that retain their free independent life 
as single cells, and are unable to band themselves 
together in associations and in the forming of a state. 

Holding firmly to this cellular-political fundamental 
idea, which forms the real fulcrum for the understand- 
ing of the cell-theory, we must touch briefly upon the 
most important modifications thatthe latter has under- 
gone in recent times. The protoplasm theory, as the 
idea most weighty in consequences, must be mentioned 
first. This was enunciated by Ferdinand Cohn in 1850, 
was further extended by Max Schulze in 1861, and was 
formulated in like manner by Lionel Beale in England 
the year after. Starting from the resemblance that 
the structure of ordinary plant-tissues in section under 
the microscope exhibits to a honeycomb, the cells of 
the former, independent but closely packed, were com- 
pared with the honey-cells of the latter, and hence took 
their name. In the one place as in the other the cell 
appears to be a closed sac or vesicle containing fluid. 
But soon it is evident that in very many cells an ex- 
ternal, firm, investing envelope, a true cell-mem- 
brane, is altogether wanting, and that the cell consists 
essentially of the colorless semi-fluid cell-contents, or 
more accurately, of t*he cell-substance alone. This cell- 
substance is made up, sometimes exclusively, some- 
times to a very great extent, of albuminoid matter, 
first recognised by Hugo Mohl, and called by him 



protoplasm, or first formative matter. Protoplasm, or 
the true cell-substance, is always a nitrogenous carbon 
compound of very complex chemical composition. It 
exists in the living cell always as a white semi-fluid 
aggregate ; but, most important of all, it seems to be the 
veritable possessor of the life-functions. It is the active 
factor in the cell-life. Protoplasm* performs the func- 
tions of nutrition and reproduction, of sensation and 
motion. Protoplasm is the true life-matter, or as 
Huxley has it, " the physical basis of life." 

Whilst protoplasm, or the living cell-substance, was 
thus brought to the front in the cell-theory, all other 
tissue elements met with in the completed organism, 
especially cell-membranes and intercellular substance, 
were, in view of this primary, active, living matter, 
relegated to the background as secondary accessory 
parts, as passive products of protoplasm. One part 
alone was an important exception to this, the cell- 
nucleus or cytoblast, on which Schwann and Schleiden 
had already laid stress. This is a smaller body, sur- 
rounded by protoplasm, to which it is very closely 
allied in chemical and physical properties, but from 
which it is still different morphologically. Regarded 
at first as a non-essential, and often absent, part of the 
cell, the nucleus was found more and more generally 
distributed and of higher and higher import. At last 
it was evident that every true cell, either through- 
out its life or at least in the earliest period of that life, 
has a nucleus, and that this latter has, in reference to 
certain life-processes, especially to cell-division, an 
im portance as great as, or even greater than, protoplasm. 
On these points the celebrated and painstaking re- 
searches of Edward Strasburger, Oscar Hertwig, Leopold 
Auerbach, Otto Butschli, and others, have yielded us 
the most valuable results. Although the function of 
the cell-nucleus is not thoroughly determined in all 
details, this much is certain, that the nucleus stands 
with and stands near protoplasm in the vanguard of 
cell-life as a most important part of the living cell. I 
was, therefore, right when in my " Generelle Morpho- 
logie " I spoke of nucleus and protoplasm as the two 
essential parts indispensable to the conception of the 


cell, and contrasted them as active constituents of the 
cell with the passive plasm products. 

A further advance in our knowledge of elementary 
organs was made by the discovery of the Monera. In 
the year 18G4, I observed, in the Mediterranean, near 
Nice, for the first time a very simple organism, whose 
whole body, not only during its development, but also 
in its perfectly developed and free-moving condition, 
consisted of a homogeneous, structureless piece of pro- 
toplasm without a nucleus, without any different parts. 
This Protogenes primordiajis for the first time gave us 
proof that there were organisms even more simple than 
the unicellular living beings whose bodies had not 
reached even the status of a single cell, but seemed as 
homogeneous as a crystal. In the following year (1865) 
two similar organisms were discovered by Cienkowski 
in fresh water, and were described as Vampyrella and 
Monas (better, Protomonas). I placed therefore in my 
"Generelle Morphologie" (vol. i., p. 133 : vol. ii., p. 22) 
under the head Monera, these very low living beings 
in which the living organism " meets us, not only in 
the simplest form actually observed, but also in the 
simplest form conceivable;" and I called attention to 
the lofty significance that they had as compared with 
all other organisms. All other living things, all plants 
and animals, and all the Protista that are neither plant 
nor animal, are made up of differentiated parts. Even 
the simplest of these, the unicellular forms, consist of 
at least two different parts, the protoplasm and the 
enclosed nucleus. The Monera alone dispense entirely 
with such a composition. Their protoplasmic body, a 
very simple living globule of jelly, has not gone even 
as far as the formation of a nucleus. They are, in 
truth, organisms without organs. All life-functions, 
nutrition, reproduction, sensation, movement, are per- 
formed by these Monera, without any different parts 
being told off for particular functions. Every fragment 
can do everything that the whole can do. Consequently, 
here, as in a crystal, every smallest particle is of homo- 
geneous chemical composition, every molecule is in its 
physiological or its chemical and physical nature like 
all the rest of the body. Hence the Monera stand on 


the boundary-line between the organic and inorganic, 
between the living and the dead. Hence are they, and 
they alone, able to give us a picture of the primordial 
origin of the former from the latter, they alone are able 
to solve for us the vast problem of the origin of life. 
The Monera alone could have been able, by intrinsic 
reproduction or ontogeny, to arise out of inorganic 
materials (** Gen. Morph.," chap, v.) 

The extraordinarily high morphological and physio- 
logical significance that attached to the Monera, on 
which I laid stress in my " Generelle Morphologie," in 
1866, I worked out further in my monograph of the 
Monera, and in the essays on the plastid theory added 
to that work (1868). I was in an especial manner 
led to this work by further observations on certain 
new Monera, that I had the opportunity of studying in 
1867 on the coast of one of the Canary Islands, Lancerotta, 
and in the Straits of Gibraltar. Certain fresh water 
Monera also, living near Jena, and observed later by 
Von Kleinenberg among others, furnished yet further 
contributions to the natural history of these exceed- 
ingly simple organisms. Most wonderful, most im- 
portant of all, was the mass of Monera belonging to the 
deep sea-beds that Huxley described in 1868, under 
the name Bathybius. This Von Bessels observed re- 
cently again (1874), on the bed of the North Polar Sea 
in living condition, and examined with especial refer- 
ence to its rhizopodic movements. In the Monera that 
were first observed, the homogeneous, formless proto- 
plasm mass of the body seems generally individualised 
to this extent, that the single pieces reach a certain size 
by growth, and then, when this is surpassed, separate 
by division into two or more pieces. In Bathybius, not 
even this commencement of individualisation is observ- 
able. Its colorless, shapeless protoplasm-body, covering 
in enormous masses the deepest abysses of the sea, has 
apparently no individualisation at all. The separate 
pieces do not seem to attain any definite size. They 
multiply according to the circumstances in which they 
are placed ; that is, they fall into fragments according 
to no definite law or plan, when their growth has 
reached the limit of adaptation to the surrounding cir- 



cumstances for the time being (Of. "Kosmos," vol. i., 
1877, "Bathybius and the Monera "). 

I have already pointed out in the " Generelle Mor- 
phologie," that the Monera (and also the so-called non- 
nucleated cells, that are met with under other circum- 
stances, and to which we shall return presently), no 
longer come within the limits of the cell-theory held 
up to the time of their discovery, and that this theory 
must, of necessity, have a corresponding extension. 
For if we take away from the idea of the cell, in its 
strictest sense, all accessory and secondary things, all 
non-essential accidental accompaniments, there still re- 
mains the idea of its composition of two parts of differ- 
ent morphological and physiological value, the outer 
cell-substance, the inner cell-nucleus. But the Monera 
do not present this differentiation, this earliest separa- 
tion of parts in the elementary organism. Their bodies 
are neither really true protoplasm nor true nucleus. 
They are rather homogeneous masses of albuminoid 
matter having the nature of both. It is at once 
cell-substance and cell-nucleus. It is, therefore, with 
wisdom called living-matter or formative matter, plasm 
or bioplasm. But all so-called non-nucleated cells, all 
elementary organisms whose active bodies consist, as in 
Monera, of plasm alone, must be separated from the 
nucleated cells, and placed apart from these under the 
name of cytods. 

Similar cytods appear in the circle of development of 
other organisms. Eclouard van Beneden, especially, was 
the first to show that the embryo of the unicellular Gre- 
garina is at first only a simple cytod. The embryonic 
globule of the Gregarina consists of homogeneous plasm. 
Later on, as a secondary phenomenon, the separation 
or differentiation takes place, as result of which the 
inner cell-nucleus is marked off from the outer cell- 
substance. The formative plasm differentiates into 
protoplasm and cytoblast. But of yet more importance 
and interest is the significant fact, that every higher 
organism at the beginning of its individual develop- 
ment is found to pass through a cytod-stage. Either 
before impregnation, or immediately after, the female 
ovum-cell loses its nucleus. The act of impregnation 



itself consists in the fusion of these non-nucleated cells 
with the spermatozoa of the male. The nucleus of the 
latter altogether, or in great part, vanishes in the fusion 
with the ovum. The product of this fusion is at first 
not a cell, but a cytod. As this non-nucleated cytod, 
with which, in reality, the organism that is the result 
of the act of reproduction begins its individual exist- 
ence, as this cytod is according to our biogenetic law, a 
repetition conditioned by heredity of the original 
Moneron parent form, I have called this embryonic 
form, Monerula. Afterwards, the plasm of this Morie- 
rula differentiates into two different substances. A part 
of the internal molecules form the nucleus, and sepa- 
rate from the surrounding protoplasm. Thus arises from 
the first cytod, the first cell. It is evident that both the 
lif e-phsenomena of these independent Monera, and these 
earliest histological differentiations in the individual 
development of the higher organisms, are of fundamen- 
tal meaning. Physiology and morphology, phylogeny 
and ontogeny, can draw thence the weightiest conclu- 
sions. For they show us, first, that commencing life 
has first to do with a formless, structureless mass, as 
homogeneous as a crystal. They show us, next, how 
such a cytod, despite the want of all organs, is able to 
perform all life-functions, nutrition and reproduction, 
sensation and movement. They give us, again, clear 
proof that life is, in its strictest sense, linked on, not to 
a body of special form, morphologically differentiated, 
made up of different organs, but to a formless substance 
with determinate physical qualities, of determinate 
chemical composition. They teach us, lastly, how such 
a cytod, made up of plasm alone, can pass by differen- 
tiation of nucleus and protoplasm into the true cell. 

As regards the cell-theory, the momentous conclusion 
follows from all this, that the cell is not, as was gene- 
rally held, the simplest, oldest, lowest elementary or- 
ganism ; but that the true, nucleated cell must arise 
from the lower, non-nucleated cytod. Cytods and cells 
are the two chief forms of the elementary organisms or 
life-units. Organic life on our globe began with the 
cytod, a simple piece of plasm. Thence, later on, the 
protoplasm and nucleus differentiated and the cell 



arose. The cytocl is the first and lower, the cell the 
second and higher form of the life-unit. I have given 
to the two together the name of plastids : for they only 
are, in truth, the plastic artificers that have the power 
to build up all the wondrous structures of organic life. 
All organic forms owe their existence to the construc- 
tive faculty of microscopic plastids. Thus the cell- 
theory widens out into the plastid-theory (Of. my bio- 
logical u Studies on Monera and other Protista," 1870). 

As now the broader conception of the plastidule has 
taken the place of the narrower conception of the cell, 
as, by consequence, the whole mysterious problem of 
life carries us back to the elementary chemical pro- 
perties of plasm, our next task is to obtain, as far as 
possible, an exhaustive knowledge of the nature of this 
most important life-matter, of this the true physical 
foundation of life. In the first place, we ought to 
demand of Chemistry that it give us details as to the 
quantitative composition and the qualitative properties 
of plasm. But, unhappily, our chemical knowledge of 
plasm is in inverse relation to its extraordinary im- 
portance. Not that there have not been many pains- 
taking attempts to unravel the enigma of the chemical 
constitution of the numerous modifications of plasm, 
protoplasm, and nucleus. But the difficulties in the 
way of these attempts are altogether abnormal and, in 
part, insurmountable. First, it is altogether impossible 
to isolate and investigate any perceptible quantity of 
plasm in a chemically pure state, because the simple 
plasm of the cytods and the protoplasm and nucleus of 
the cells are so intimately mixed with other substances 
that have been formed out of them, and are in small 
quantities interspersed amongst, interwoven with other 
tissue elements, as cell-membranes and intercellular 
substance. But further, all modifications of plasm are 
decomposable and changeable in a far greater degree 
than other allied albuminoid bodies. Above all, it 
must be kept in mind that although the modifications 
and varieties of plasm bodies are endless in number 
and in variety, these are as nothing in comparison with 
the variations' in quantitative composition. The coarse, 
rude methods of our chemistry of to-day are far enough 



from the solution of a problem of such delicacy and 
difficulty. But that very variability beyond limit, that 
very readiness of decomposition and mobility of the 
atoms in the plasm-molecules, are of deepest signi- 
ficance in the study of Evolution. For they explain to 
us how, under those physical and chemical influences 
of the environment, endless in number and in variety, 
that occur in the nutrition of the plasm, this latter 
undergoes slight alterations, endless in number and 
variety, and thus can evolve into the most diverse 
organic forms. 

From the physiological and chemical side, therefore, 
it is possible to regard all plasm-bodies as a single 
great group of allied compounds, and to class them 
together as the plasm-group. In this group we may 
perhaps mark off : 1. the archiplasm as the oldest life- 
substance, arising originally directly as the result of 
ontogeny ; 2. the monoplasm, as the body-material of 
the cytods living at the present time ; this probably 
varied more or less from the archiplasm ; 3. protoplasm, 
or the true cell-substance ; 4. nuclein or coccoplasm, the 
substance of the nucleus, as the whole, chemically dif- 
ferent, nitrogenous basis of the cell-nucleus may be called. 
Although protoplasm and coccoplasm are very nearly 
allied and most intimately related, they appear to be 
essentially different, possessing characteristic and, to 
some extent, opposed qualities, that are not distinctly 
marked off one from another in archiplasm and mono- 

Everything that is essential of that which we know 
as to the plasm-group may be summed up as follows. 
The plasm-group is a part of the larger group of albu- 
minoids or proteids. Like the rest of the albumin- 
compounds, the plasm bodies are distinguished by an 
extraordinarily complex composition of their atoms. 
In every molecule at least five elements are present, 
and on an average the percentage composition is 52-55 
carbon, 6-7 hydrogen, 15-17 nitrogen, 21-23 oxygen, 
1-2 sulphur. The way in which the atoms of these 
elements are united in each plasm-molecule to form a 
chemical unity is, without doubt, a very complex and 
special question, and stands in direct causal connexion 


with the vital properties of these wonderful compounds. 
For the sum of those physical and chemical properties 
that we name with the word "life " is clearly, in the 
ultimate analysis, conditioned by the molecular struc- 
ture of the plasm, and this again is, on my carbon- 
theory, referable to the peculiar and very marvellous 
property of carbon, by virtue of which it enters into 
the most complex and most unstable compounds with 
the other elements I have named. With great wisdom 
has recent chemistry called the collective study of the 
so-called organic bodies, or, what was in earlier times 
called organic chemistry, the chemistry of the carbon 
compounds. But with equal justice I have regarded 
the chemical and physical nature of carbon as the 
ultimate cause of the peculiarities that distinguish the 
organic from the inorganic, in a word, as the ultimate 
basis of life. If this carbon-theory is rejected as an 
arbitrary and phantastic dogma, denial is made of the 
causal connexion between the chemical composition 
of plasm and the physical phenomena that I call in a 
word life functions. 

Amongst the physical properties of plasm the most 
noticeable is its marked power of imbibition, the faculty 
of absorbing water in variable, and often in very con- 
siderable, quantities, and of placing it between its 
molecules. Hence results that peculiar soft yet dense 
condition of all living tissues, which we call a condi- 
tion of semi-fluid aggregation. It seems to be a neces- 
sary pre-condition of all the complex molecular move 
ments whose totality we call life. The readiness with 
which plasm under varying external conditions absorbs 
or gives out water and watery solutions is, therefore, of 
special significance, not less than the extraordinary 
inclination of most kinds of plasm to commingle with 
other carbon compounds (fats, for example) and with 
salts. These and many other peculiarities of the plasm- 
group prove that we have here to do with carbon-com- 
pounds, whose molecules differ from all others in an 
unwonted mobility and changeableness, instability and 
many-sided affinity. These plasm-molecules, at every 
farther investigation of elementary conditions, are 
eternally forcing themselves to the front. With them 


we have to do in our perigenesis as the really active 
elemental factors. 

The plasm-molecules or plastidules, as we will call 
them in brief, after Elsberg, possess, in the first place, 
all the properties which Physics usually ascribes to 
hypothetic molecules or " compounded atoms." There- 
fore every plastidule is not further decomposable into 
smaller plastidules, but can only be broken up into its 
constituent atoms, and, of course, only into atoms each 
of which contains the five elements already named. 
The plastidules are probably always surrounded by 
water-envelopes, and the greater or less thickness of 
these, which at once separate and connect adjacent 
plastidules, determines the softer or firmer plasm into 
whose composition they enter. Probably the plastidules 
are so small, that the smallest piece of plasm which we 
are able to see by aid of our best microscopes, contains 
an enormous number of plastidules. That which holds 
of the original, simple plasm or archiplasm, naturally 
holds in general as to protoplasm and coccoplasm that 
arise by differentiation of the former. The protoplasm 
molecules may be named, for short, plasmodules, the 
nucleus-molecules, coccodules. The same physical pro- 
perties and physiological functions exhibited by the 
homogeneous plastidules, in the homogeneous plasm of 
the cytods, are found to be common to the plasmodules 
and coccodules in the cells. The plasmodules and 
coccodules have arisen, later on, by differentiation from 
the plastidules. 

Besides these general physical properties, which 
Physics and Chemistry ascribe to-day to the molecules 
of matter in general, the plastidules have special attri- 
butes, exclusively theirs, that are, in common parlance, 
vital properties. By these the living is in a general 
way separated from the dead, the organic from the in- 
organic, using these words in the ordinary fashion. 
Every more exact, and more detailed comparison of 
organisms with inorganic things, based on the broad 
experimental basis of newly discovered facts, and most 
of all the unprejudiced comparison of the Monera and 
crystals are teaching us, now-a-days, that the gap be- 
tween these two main groups of natural bodies is much 


narrower than is commonly thought. I can refer in 
this connexion to the detailed comparison of organisms 
and inorganic things that I have given in the fifth 
chapter of my " Generelie Morphologie," (vol. i., pp. Ill 
— 166). Many properties that our superficial ideas of 
nature have hitherto ascribed to organisms alone, be- 
long also to the inorganic, and are, in fact, the com- 
mon property of natural bodies, or to speak more 
exactly, the common property of all atoms of all the 
smallest, separate particles of bodies which modern 
chemistry unanimously regards as the ultimate parts of 
all bodies. 

As the views of chemists and physicists, as to the 
nature of atoms, and of the aether existent between the 
masses of atoms are in accord, so also certain elemen- 
tary ideas as to their essential nature, have now ob- 
tained general acceptance. We must hold that atoms 
are the smallest separate particles of masses, having an 
unalterable nature, separated one from another by hypo- 
thetical aether. Every atom has an inherent sum of 
force, and is in this sense gifted with a soul. Without 
the acceptance of an atom-soul, the commonest, the 
most general phaanomena of chemistry are inexplicable. 
Pleasure and displeasure, desire and loathing, attraction 
and repulsion, must be common to all masses of atoms; 
for the movements of atoms which must occur in the 
formation and decomposition of every chemical com- 
pound, are only explicable if we impute to them sensa- 
tion and will. On what ground does the generally 
accepted chemical teaching of "affinity" in bodies rest 
except on the unconscious supposition, that in fact the 
attracting and repelling atoms are possessed with 
definite inclinations, and that they follow these sen- 
sations or impulses, have also the will and the ability to 
move to or from each other. The whole truth is em- 
bodied in that which Goethe has said in his " Wahlver- 
wandtschaften," as to these relations, when he traces 
them up from the elementary soul-life of atoms to the 
very complex soul-life of man ; and when in that 
classic romance affinity is laid down as the sole spring 
of human actions, and of the history of the world which 
is the sum of these, the mechanical nature of the most 



complex organic processes is thus denoted in unmistak- 
able manner by the great thinker and poet. 

And if the will of man and higher animals seems to 
be free as opposed to the fixed will of atoms, herein 
lies an illusion due to the very complex movements of 
the will in the one case, and the exceeding simplicity 
of them in the other. The atom everywhere, and at all 
times, wills in the same fashion, because its disposi- 
tion with regard to the atom of every other element is 
a constant, is unalterably fixed : every one of its move- 
ments, therefore, is determined by this. On the other 
hand, the disposition and arbitrary movement of the 
higher organisms appear to be free and unconditioned, 
because in the incessant change of material, the atoms 
are continually altering their relative positions and 
modes of combination, and as a consequence the total 
result of the innumerable w r ill movements of the con- 
stituent atoms is of the greatest complexity, and is 
changeable without end. Hence we are " a sport for 
every breath of wind." 

Whilst from the mechanical standpoint of Monism 
we regard all matter as possessing a soul, every mass of 
atoms as provided with a constant and eternal atom- 
soul, we are not afraid of having the reproach of 
materialism hurled at us. For this our monistic posi- 
tion is as far from a one-sided materialism as from the 
emptiness of spiritualism. In it alone are we able to 
find the reconciliation of the crude atomic and the 
empty dynamic conceptions of the universe, which 
have up to the present time been in such violent con- 
tention, and which in their limited view are each 
dualistic. As the mass of the atom is indestructible 
and unchangeable, so the soul of the atom indissolubly 
bound up therewith is eternal, is immortal. Transitory 
are the numberless and eternally changing combina- 
tions of the atoms, the unending manifold modifications 
in which atoms unite to build up molecules, molecules 
to build up crystals or plastids, plastids to build up 
organisms. This monistic conception of the atom 
alone is in harmony with the great law of the con- 
servation of energy and the conservation of matter 
— that law which the natural philosophy of the 


present age rightly regards as its inalienable funda- 

If, therefore, we regard all matter as in possession of 
a soul, every atom as gifted with sensation and will, 
we can no longer look upon these two properties, as 
men generally look upon them, as exclusively the 
prerogative of organisms. We have to look for other 
qualities that may separate the organic from the inor- 
ganic, the plastidules from other molecules, and that 
may constitute the essence of life in its narrower sense. 
The most important of these qualities seems to us the 
faculty of reproduction, or of memory, that is at work 
in every event of development, and especially in the 
reproduction of organisms. All plastidules have 
memory. This faculty is wanting in all other mole- 

In a notable treatise, as carefully thought out as 
clearly written, on " Memory as a general function of 
organised matter," Ewald Hering in 1870 has discussed 
this momentous question so thoroughly that we need 
not enter in this place into any detailed proof. We 
need but refer to his treatise. We are in truth con- 
vinced by the profound thought that if we ignore the 
idea of an unconscious memory as a property of living 
matter, the most important life-functions are, as a rule, 
inexplicable. The faculties of ideation, of thinking, of 
consciousness, of use and habit, of nutrition and re- 
production glide into the function of unconscious 
memory, whose action is of far more general im- 
portance than is that of conscious memory. Hering 
says rightly enough "that it is to memory we owe 
almost all we have and are." 

On one point alone we must join issue with the 
statements of Hering, or rather we must limit them a 
little more definitely. We look upon memory not so 
much as a general function of all organised matter, but 
only as a function of the really living, the plasm. All 
plasm-products, everything formed of protoplasm and 
nucleus, all the inactive organised parts of the organism, 
are destitute of memory, even as is inorganic matter. 
Strictly speaking, on our plastid theory only the group 
of plasm-bodies is gifted with memory ; the plastidules 


alone are reproductive, and this unconscious memory 
of the plastidules conditions the characteristic move- 
ments of the molecules of the plastidules. 

The difference which memory, or the power of re- 
production in the plastidules, determines between the 
organic and inorganic, shows itself first in the different 
method of their growth, and this is clearly determined 
by their different condition of aggregation. The inor- 
ganic grows by apposition, or by the external addition 
of molecules. The organic grows by intussusception, 
or the internal deposition of molecules. The most 
perfect inorganic individual, the crystal, grows by par- 
ticles on particles, depositing externally on the solid 
crystalline body already in existence, The least perfect 
organic individual, the Moneron, grows by particles, 
penetrating from without into the interior, and becom- 
ing assimilated by the semi-fluid plasm-bodies. This 
assimilation is of such a nature that new plastidules 
are being formed from the nutritive fluid, that is, 
absorbed, and are being deposited between pre-existing 
plastidules. The semi-fluid condition of the organic 
matter is the pre-condition of this peculiar growth, and 
the molecular structure of the carbon compounds its 
true cause. This growth by intussusception, occurring 
in all organisms, wanting in all organic bodies, is the 
explanation both of that nutrition and that transforma- 
tion of matter which distinguish the former from the 
latter. Finally, this growth by intussusception is the 
chief circumstance that conditions that life-phseno- 
menon, which stands out prominently as the most im- 
portant factor in organic evolution, and of which we 
must, therefore, treat next : reproduction and its com- 
panion, heredity. 

It is beyond dispute that reproduction, more than 
any other function, marks off the organic from the 
inorganic. For through reproduction alone, through 
heredity, which we regard as an essential and integral 
part of reproduction, is the preservation of the organic 
species and phyla possible, those species and phyja that 
continue to exist in the linked chain of generation on 
generation, despite the continual vicissitudes of indi- 
viduals. Since no inorganic body is capable of repro- 


duction, inorganic nature has nothing of the ancestral 
history, the phylogeny which characterises the organic 
world. The study of reproduction or gonology is, 
therefore, the essential starting-point for the due under- 
standing of phylogeny. 

What is reproduction ? To give a correct answer to 
this question we must first of all divest ourselves of the 
ordinary idea that the union of the two sexes is the 
most important and most necessary event in reproduc- 
tion. This idea, based upon the general method of 
reproduction of individuals in man and in the higher 
animals and plants, is seen at once to be incorrect, as 
soon as we think of the asexual processes of reproduc- 
tion, endless in their extent, that everywhere and at all 
times are taking place in the multiplication of the 
plastids. On the whole, sexual or amphigonic repro- 
duction, with its special peculiarities, appears to be 
only a special case among the multitude of events 
which we group together as reproduction or procreation ; 
events which in by far the greatest number of cases are 
asexual in their nature. All the countless myriads of 
cells that make up the body of every higher animal, of 
every higher plant, arise not by sexual, but by asexual 
reproduction, by division. All, or at least the majority, 
of the many unicellular beings that stand on the limit 
of the animal and vegetable kingdoms, and that we 
•class together as Protista, increase in number not by 
sexual but by asexual reproduction. But even many 
higher animals and plants, that have sexual reproduc- 
tion, multiply in addition in asexual ways, by division, 
by gemmation, by spore formation. If we reflect how 
everywhere and at all times masses of plastids are 
perishing, and are restored by division and by bud- 
ding, it is evident that asexual reproduction is the 
general rub, and sexual a relatively rare exception to 
that rule. Certainly we should be making an estimate 
too low rather than too high, if we say that, on an 
average, for every simple sexual act of reproduction in 
nature more than a thousand, probably more than a 
million, asexual ones occur. 

But now it is evident that the simplest forms of 
asexual or monogonic reproduction, first fission, next 


gemmation, give us the clearest explanation as to the 
nature of reproduction, and are our guides to the far 
more difficult and more complex sexual methods. Turn- 
ing to those simplest forms of monogony we find as 
simplest answer to our question. Reproduction is the 
growth of the individual beyond its individual mass. 
If a very simple plastid, or homogeneous Moneron 
has grown up to a certain bulk, the structureless 
plasm-body splits as its growth continues into two 
similar halves, inasmuch as the cohesion of the plasti- 
dule no longer is sufficient to hold the whole mass in 

In like manner, every ordinary cell-division depends 
essentially upon a growth continued beyond the indivi- 
dual mass of the particular cell. The remarkable de- 
tails of the process by which two similar daughter cells 
arise from one mother cell, have been worked out by 
Auerbach, Biitschli, Hertwig and Strasburger. That in 
these cases, the two daughter cells resembling one the 
other, have inherited the nature of their common 
mother cell seems easily understandable, for they are 
like halves thereof, and the molecular movements of 
the plastidules must be the same in the former as in the 
latter. Heredity here appears as a simple, necessary 
consequence of division. In like manner, it gives us 
here the deepest law of its nature. Heredity is the 
transmission of the motion of the plastidules, a repro- 
duction of the individual motion of the plastidules, of 
the mother-plastid in the daughter-plastid. 

But the conditions under which the two similar halves 
pursue their individual lives are always more or less 
diverse. In especial, the complex relationships that 
in the struggle for existence condition the plastids as 
well as the multicellular organisms, are ever peculiar 
for each individual. Whilst these diverse conditions 
of existence are influencing the elementary organism,, 
they affect its primary nutrition, and bring about a 
partial alteration in the primary motion of the plasti- 
dules. This alteration or variation we name adapta- 
tion. Adaptation is alteration of the motion of the 
plastidules, in consequence of which the plastid ac- 
quires new properties. If, later on, the two daughter- 



plastids that have arisen from division of one plastid 
again increase in size, and after passing the limits of 
their individual growth again split, by fission, into two, 
these four grandchildren will no longer be so alike as 
were their two mother-plastids. It is true that they 
will have inherited from these the greater part of the 
properties which the two received from the original 
grandmother. But also a part of those peculiarities 
will be noticeable which each of the two mothers ac- 
quired during its individual life, and finally each of 
the four grandchildren will itself acquire new peculi- 
arities in the course of its individual existence. Small, 
and without significance as these new acquisitions may 
seem in each single case, yet it is clear that finally, in 
the lengthy chain of many generations, they may accu- 
mulate, and sum up to very notable deviations from the 
movements of the plastidules in the original parent 

The inheritance of variations, on which the whole 
evolution of the phylum depends, has full play in the 
life of the plastids, and produces an endless number of 
different individual motions of the plastidules. Every 
later movement of plastidules — or in other words, the 
life of every plastid later on, be it cytod or cell — con- 
sists, therefore, on the one hand of the vastly prepon- 
derating set of old motions that have been maintained 
from generation to generation by heredity, on the other 
hand of a smaller part consisting of new motions ac- 
quired by adaptation. All these variations of the 
plastidules are naturally conditioned by the intussus- 
ception of atoms ; and in the endless complexity and 
variety of the atomic composition of the plastidules, in 
their extraordinary instability and tendency to decom- 
pose, an unlimited field for the production of new forms 
is offered to adaptation. 

In transferring the teaching of Lamarck as to the in- 
heritance of variations — the most important presup- 
position in the Darwinian theory — from the large 
multicellular animals and plants in whom it is seen 
palpably, before our very eyes, to the plastids (cytods 
and cells), and from these again to the plastidules that 
make up the plastids, for these last the same conse- 


quences, of course, hold that the selection-theory 
grants to the former. That the struggle for existence 
obtains amongst the molecules was first made clear by 
Pfaundler in 1870 — that it obtains in the strictest sense, 
and most of all, among the active plastidules. Those 
plastidules which best adapt themselves to the external 
conditions of existence, i.e., those which most easily 
take up the fluid nutritive matter penetrating from 
without, and thus accomplish most readily the conse- 
quent intussusception of atoms will, of course, have 
best assimilation, and thus by reproduction of plastids 
acquire superiority. 

The next consequence of Natural Selection in the 
struggle for existence, is the increasing differentiation 
of forms, which Darwin has called " divergence of 
character." Its best known form is the division of 
labor or polymorphism of persons. It is well known 
that in human life division of labor affords the best 
measure of the degree of civilisation attained. The 
same principle holds in regard to the wonderful civi- 
lised states of the ants, bees, termites, etc. Further, 
comparative anatomy shows us that the physiological 
perfection or the grade of evolution of every higher 
animal and of every higher plant, is conditioned by the 
specialisation of its organs. The complex machinery, 
e.g., which constitutes the higher Vertebrate with its 
nerves and sense-organs, muscles and bones, alimentary 
canal and blood-vessels, glands and reproductive organs, 
is determined by the specialisation of all these organs, 
and of their separate parts, a specialisation now advanced 
to an extraordinary degree, but none the less acquired 
gradually and slowly in the struggle for existence dur- 
ing the past. 

But specialisation or division of labor in organs, de- 
pends again upon that of plastids, of cytods and cells. 
The different tissues, that give to each origin its phy- 
siological peculiarities, are made up of different kinds 
of cells, those of nerve, muscle, bone, glands, alimen- 
tary canal, sexual organs, etc. The manner in which 
all these different kinds of cells have arisen by speciali- 
sation from one single, simple, original cell-form is 
shown us to-day by the individual development of the 


ovum of every higher animal. For the impregnated 
ovum-cell split up, at first, by repeated division into a 
great number of very simple, similar cells. From these 
morula-cells next arise the two primitive germinal 
layers of the gastrula, and this differentiation into two 
different cell-layers is the first commencement of his- 
tological specialisation. When the cells of the outer 
layer or of the ectoderm differentiate still further into 
cells of skin, nerve, muscle, etc., and when the cells of 
the inner layer or endoderm give rise by differentiation 
to cells of the alimentary canal and of glands, the for- 
mation of tissues, or histological differentiation, results 
on which the upbuilding of the various organs depends. 
But the ontogenetic specialisation of the cells as we are 
able to trace it out, step by step, under the microscope, 
is only on our fundamental law of biogeny the repeti- 
tion of the slow phylogenetic histogeny, or tissue 
building that was originally conditioned by the active 
specialisation of the cells. 

In what way is this specialisation of the plastids 
possible ? Clearly only by the conditioning specialisa- 
tion of the plastidules. For in exactly the same way, 
and according to exactly the same laws by, and accord- 
ing to which, the civilised state is conditioned by the 
division of labor of its citizens, is the high organisation 
of the human body by that of its organs, and these 
again by the division of labor among their constituent 
cells ; this last also is effected in like manner by the 
division of labor among the plastidules, and arises ac- 
cording to the same laws from the interaction of here- 
dity and adaptation in the struggle for existence. The 
morphological and physiological peculiarities by which 
every nerve-cell, every muscle-cell, every alimentary 
canal cell, as such, is characterised, are dependent 
simply and solely upon the fact that their constituent 
plastidules have differentiated to a greater or less extent, 
and thus have given rise to different kinds of plasm. 
Complex and heterogeneous as may be the molecular 
structure of the plasm, and its combination in the 
various kinds of cells with different sorts of plasm- 
products, none the less they all are proved to originate 
from the similar cells of the morula, just as these 


originated from the impregnated ovum-cell. The 
phylogenetic primary specialisation of the plastidules 
is, on the biogenetic fundamental law, still repeated at 
the present day in the ontogenetic differentiation of the 

One particular form of this histological specialisation 
deserves in this place our closer attention, that is, the 
sexual differentiation. As we have already remarked, 
sexual reproduction does not possess nearly the 
great general significance that is ascribed to it at the 
present time by the majority of people. It is the more 
necessary to insist upon this because in the first place 
this function is veiled with the mystic cloak, as if it 
were a supernatural or very mysterious function, and 
in the second place so many prominent scientific men 
over-estimate out of all proportion the meaning of this 
phsenomenon as regards the study of development. In 
the first place, then, it must be laid down clearly that 
a large number of the lowest organisms, e.g., the chaotic 
division of the Protista, many Protophyta and Protozoa 
know, as a rule, nothing of sexual reproduction, but 
reproduce themselves solely by asexual methods (most 
generally by simple fission, but in addition by bud-and- 
spore formation). Secondly, it should be stated that no 
sharp line of demarcation exists between sexual and 
asexual reproduction (amphigony and monogony). To 
this the interchanging conjugation and copulation that 
occur in many of the lowest organisms bear witness. 
In the third place, the wide distribution of partheno- 
genesis among very different groups of higher animals 
and plants is very instructive. It is evident that these 
have arisen from ancestors that were sexually different. 
Yet in the course of time the males have become un- 
necessary and have disappeared. No less instructive, 
in the fourth place, is the frequent connexion of sexual 
and asexual reproduction in the alternations of genera- 
tions in one and the same species. Finally, in the fifth 
place, the essential part of sexual reproduction loses all 
wonder and mystery when we abstract from it all 
non-essential and secondary accidents, and fix our 
attention firmly and only on the histological essential 
of the process. For then sexual reproduction is no 



other than the conjunction of two plastids that have 
developed in very different directions, as result of an 
extraordinary specialisation of their plastidules. 

Thus, in fact, the dark mystery of sexual reproduc- 
tion is cleared up in the simplest manner, and the 
" wondrous riddle " of love that moves the world solved 
in soberest fashion. Of course we must eliminate all 
those manifold and remarkable sex-arrangements that 
have been slowly and gradually acquired by the higher 
plants and animals, in part under the general influence 
of Natural Selection, in part by the special working of 
Sexual Selection. Originally we meet with nothing- 
more than two different kinds of cells ; the female egg- 
cells, the male sperm-cells. These frequently do not 
even appear in special organs, but lie separately dis- 
persed in different tissues, the egg-cells between the 
epithelial cells of the alimentary canal, the sperm-cells 
between the epidermal cells of the skin. This is seen 
in the Gastrceada, Spongida, many Hydrozoa, etc. All 
that occurs in the way of sexual union in these cases is 
that these two kinds of cells, set free from their con- 
nexion with the multicellular organism, and meeting, 
haphazard, in the water, apply themselves each to each, 
and blend one with the other into a single plastid. The 
internal attraction, conditioned by the chemical affinity 
of the two living cells, draws them inevitably together. 
The new cell thence arising is the child of the mother 
germ-cell and the father sperm-cell : it consists of the 
conjoined bodies of both. If we trace out this very 
important but very simple fundamental process of 
amphigony yet further, we find that from it a complete 
and intimate commingling of the plastidules results, a 
thorough blending of the different molecular move- 
ments in the two plastids. Generally a partial or 
complete vanishing of the nuclei seems to precede, or 
in some cases to follow, the blending of the two sexual 
cells. Hence the newly produced individual is at first 
no cell but a cytod, and turns into a cell later by the 
re-formation of its nucleus. We have called this cytod, 
monerula, and the first cell, cytula. It is evident that 
the individual plastidule motions that are inherent in 
this first plastid, and condition all its further develop- 




ment, are the resultant of the two different plastidule 
motions of the female egg-plastid and the male sperm- 
plastid. If we look upon these latter as the two sides 
in the parallelogram of forces, the plastidule motion of 
the monerula and of the cytula resulting thence is the 
diagonal of that parallelogram. In this way there is a 
simple explanation of the fact, well known in amphi- 
gonic heredity, that the child inherits many properties 
of both parents. The primary life-movements of the 
child are the diagonal between those of the mother and 
of the father. 

Considered from the purely morphological side, that 
blending of the two kinds of sexual cells that is the 
sole essential in sexual reproduction, is from beginning 
to end no peculiar process. It falls, rather, under the 
broader idea of the concrescence or growing together of 
plastidules, a histological process, that we have already 
seen very general under many and various modifica- 
tions, e.g., in the formation of the plasmodia of the 
Monera and Myxomycetes, in the formation of the reti- 
culate tissues, the blending of the stellate cells of 
muscle, of nerve, of connective tissue, etc. But of 
especial value in this connexion is the so-called conju- 
gation of two apparently similar cells, that precedes in 
many Protista (Protophyta and Protozoa) the asexual 
multiplication by division (Gregarinida, Infusoria, 
Diatomaceae, Desmidiacese, etc.). We ought to look 
upon this conjugation of two similar plastids as the 
first shadowing forth of sexual differentiation, or as 
the transition from asexual to sexual reproduction. 
As, according to the well-known experiments of inter- 
breeding, a certain degree of difference between the two 
sexual individuals is of distinct advantage to the result 
of their union and the fertility of their progeny, 
Natural Selection will favor dissimilarity between the 
two conjugating plastids, and will, by gradual accumu- 
lation and identification of their individual peculiarities, 
lead them on by degrees to the notable condition that 
we see in the different composition of the large amoe- 
boid ovum-cells and the small flagellate sperm-cells in 
the majority of animals. This also is but a special and 
strongly marked form of division of labor. 


If again we call to mind, that considered as a whole, 
reproduction is nothing else than a growth of the indivi- 
dual beyond its individual mass, we shall regard every 
conjunction of two similar cells which is known as 
conjugation, and has taken the first phylogenetic step 
towards sexual differentiation, as only a special form of 
growth. Whilst in the ordinary simple process of 
asexual reproduction, the preceding and determinal 
growth (general in fission, partial in gemmation) pro- 
ceeds slowly and gradually, in conjugation this takes 
place swiftly, suddenly. Once again the mystery of 
sexual reproduction is traceable back to a special form 
of growth and of division of labor. 

The idea of sexual reproduction here given seems, 
as far as regards the lower, simpler forms, so evident, 
that it requires no further proof. But it gives us also 
the key to the understanding of the higher, more 
complex forms, which do not at first sight seem so 
thoroughly explained by it. To this end it is necessary 
that we, first, understand the physiological individu- 
ality of the plastid-life, and the active significance of 
the plastidules on which that depends, and that we, 
secondly, give to the idea of alternation of generations a 
far wider extension, and more general application than 
has been hitherto given. This alternation of genera- 
tions, that we name briefly, after Owen, metagenesis, 
depends on the regular, periodic alternation of two or 
more different generations, whereof one produces its 
offspring asexually, the other sexually. With this 
periodic alteration in the method of reproduction is 
bound up a more or less complete specialisation of per- 
sons (or in plants of the shoots) that is often evident in 
a very striking difference in form and organisation. 
Thus we see, e.g., that from the spores of the fern no 
fern arises, out a prothallium, a new form of plant 
without stem or leaves, resembling in all essentials a 
liverwort. This prothallium is sexual. It forms ova 
and sperm-cells, and from these arises a new cell, 
the cytula. As the cytula undergoes repeated division 
a little plant is formed, that develops, by differentiation 
of stem and leaves, into a fern. On the under side of 
the leaves of this fern are formed later on the brown 

R 2 


masses of spores. We see a like alternation of genera- 
tions in very many of the lower animals. Thus, from 
the fertilised eggs of most Medusae, no Medusa develops, 
but a stationary hydroid-polyp, of quite another struc- 
ture. This in its turn gives rise (by asexual gemmation) 
to the free swimming Medusae, that are sexual. The 
Aphides and many small Crustacea (e.g., the Daphnidae) 
reproduce themselves during the summer asexually, by 
parthenogenesis, through the medium of unimpregnated 
egg-cells or spores. Later on, in the autumn, appears a 
generation, sexually differentiated, with males and 
females, and from the fertilised eggs of this generation 
arises the first asexual generation in the spring. 

But if we now classify our plastids as autonomous 
" elementary-organisms," possessing morphological and 
physiological independence, and if we consider the 
individual process of development from the histological 
standpoint of the plastid-theory, we shall, on a survey 
of the above-quoted facts, be led to the idea that alter- 
nation of generations or metagenesis is in reality a very 
widely spread phsenomenon. For in the individual 
development of every multicellular animal, of every 
multicellular plant, appears, first, a generation of sexual 
plastids represented by the female ovum-cell, and the 
male sperm-cell. As result of their union arises a cell, the 
cytula, and this produces, in asexual fashion, by repeated 
division, the generation of similar cells that form at 
length the morula, and the blastula that arises from it. 
For the first time division of labor makes its appearance 
among the similar cells of the blastula generation. They 
separate into two kinds of cells, those of the inner or 
vegetative, those of the outer or animal layer of the blas- 
toderm. Each of these, again, by repeated division gives 
rise to many generations, and in these the specialisation 
of the cells is the more marked, the more complete the 
organisation of the fully developed individual. All 
the innumerable generations of dissimilar cells that 
make up tissues and organs multiply asexually by 
division. Two only of these polymorphic cell genera- 
tions differentiate sexually, the ova and the sperm-cells. 
When these, later on, in the sexual act of reproduction 
come again into union, we have re-reached the begin- 


ning of the reproductive circle at which we started. 
The reversion or atavism of the plastids has led us back 
once more to the cytula. Essentially, the individual 
development of every multicellular animal or plant, 
that is reproduced by hypogenesis, i.e., without any 
alternation of generations of the persons by a sexual act 
of reproduction, consists in reality of a very complex 
alternation of its constituent cells. The difference lies 
only in this, that the latter remain in close contact one 
with the other in the multicellular organism, whilst 
the persons, as representatives of the different genera- 
tions in true metagenesis are widely separated one from 
another and are free. To emphasise this difference, I 
have called the alternation of reproduction in the plas- 
tids, strophogenesis (" Gen. Morph." ii., 106). The 
word metagenesis is left for the alternation of repro- 
duction in persons, limited to individuals wholly inde- 
pendent and physiologically free. How non-essential, 
for the rest, this distinction is, the Siphonophora show. 
In these the same persons, widely differentiated by 
division of labor, remain united on one stock that in 
other Hydro-medusae lead separate and independent 
lives. The specialisation of persons that we meet with 
in these animals as in the civilised states of the ants, 
bees, termites and men, is, considered in itself, only on 
a large scale the same as the specialisation of the plas- 
tids in the course of strophogenesis is on a small scale. 
The latter is, in essentials, no other process than that 
specialisation of the plastidule that we have already 
considered. This is the elementary factor of advancing 
organic evolution, the ever increasing improvement 
and variation of organic forms. Here, as ever, the 
microcosm is the image of the macrocosm. 

If we try to find a unifying general point of view on 
a Monistic basis, for the manifold and wondrous events 
of organic reproduction and development thus lightly 
touched on here, this can, at all events, only be sought 
in the domain of the study of motion, or of mechanics 
in its more rigid sense. For all the processes of the 
universe that are known to us in all their limitless ex- 
panse, the evolution of the solar system and the planets 
after Kant, the organic evolution of this globe after 


Lyell, its organic evolution after Darwin, are one and 
all, of necessity, conditioned by fixed, unalterable me- 
chanical laws. And like the whole of the evolution of 
organic nature on our earth, like the ancestral history 
of the plant and animal kingdoms, the history of the 
evolution of mankind, and of each individual man, is 
governed by the same fixed laws of dynamics. The 
sole difference lies in this, that the process of evolution 
in organic nature is as a whole and in details infinitely 
more complex, and therefore more difficult to grasp, 
than is the same process in the organic world. But the 
one, as the other, depends essentially only on molar 
motion, and molar motion is entirely referable to at- 
traction and repulsion of molecules, of the atoms which 
make up these, and of the aether that envelopes atoms. 

The biogenetic process, as we may name in brief the 
totality of the organic movements of development on 
our planet, is far too complex in its details, the number, 
variety and complication of all the particulars that 
compose it are far too vast, for it to be possible, 
with our imperfect knowledge of them all, to follow 
step by step its rigidly mechanical course. None the 
less we may hold that we have already gained a Monis- 
tic insight, not unsatisfactory, into its true nature. 
The preliminary to this is the recognition of that bio- 
genetic fundamental law, which by its proof of the 
causal nexus between ontogeny and phylogeny, alone 
seems capable of dissipating the clouds that beleaguer 
all branches of biogeny. The formulation of that in- 
timate causal connexion between the history of the 
embryo and the history of the race, is only possible to 
him that is unblinded by prejudice, is acquainted with 
the facts of organic evolution, and is capable of a philo- 
sophical judgment on their meaning. 

But if we desire to inquire still more closely into the 
mechanics of the biogenetic process, we must descend 
into the dark depths of the plastid life, and seek for the 
true and effective cause thereof in the motion of the 
plastidule. Here, also, finally, the question must be 
asked whether we are in a position to form a preli- 
minary hypothesis, satisfactory in its nature as to the 
peculiar nature of this molecular motion of the plasti- 



dules, hidden from our actual observation, by means of 
the study of analogous motor phenomena. Our hypo- 
thesis of perigenesis aims at an approximate answer to 
this question. 

If, in the first place, we survey from the highest and 
most comprehensive point of view the totality of the 
phenomena of development already known, the most 
general result is a conviction that the biogenetic pro- 
cess runs its course as a periodic movement. We find 
its clearest analogy in the similitude of a complex 
undulatory movement. Confining ourselves at first 
only to the facts most directly within our ken and 
most incontrovertible, we are able to trace this truth in 
our own ancestry. It is the same if we limit ourselves 
to the so-called historic period, in which man is, 
beyond a doubt, followed by man ; or if we trace out 
our ancestral series further and further down through 
the sub-kingdom of the Vertebrata to Amphioxus, 
through the Invertebrata down to Gastraea, and at last 
to Amoeba and to Moneron. To us, proceeding thus, 
the movement of development in our ancestral series 
is representable under the simple similitude of a series 
of undulations, in which the individual life of each 
person corresponds with a single wave. 

But if we cease to confine our attention to the series 
of our direct ancestors, and extending it, take into view 
all our blood-relations, we can, without doubt, picture 
their connexion in the simple form of a genealogical 
tree. As regards the undulatory movement of con- 
nected evolution, we may represent the movement of 
development of each individual person on this tree by 
a wave. Thus the genealogical tree, as a whole, pre- 
sents the picture of a complex wave-motion, a ramified 
undulation. Whatever ancestral form we may take to 
be the parent of any whole group on the genealogical 
tree, whose members are connected by descent, or as 
the parent of any part of such a group, it will always 
be as the point of origin of a connected undulatory 
movement that, as the branches and twigs of the 
genealogical tree, ramifies in all directions. 

We find the same picture of a ramified undulation, 
that is furnished us in little by the history of the 


evolution of every human family, by the genealogy of 
every dynasty on a larger scale, when we study the 
natural system of organisms in the light of the theory 
of descent. For in every considerable group of related 
plants and animals, as in every human family, " all 
forms are alike : yet none resembles its fellow." The 
" secret law," the " holy riddle," to which this choir of 
forms points, as Goethe has it, is that transmitted 
motion of development on which affinity depends. 
Hence the natural system is none other than the true 
genealogical tree of related species, and its every 
branch and twig represents a smaller or larger group 
of descendants from one common ancestral form. This 
unity of origin connects all the forms of a class, of an 
order, etc. When every class is divisible into different 
orders, every order into several families, every family 
into genera, every genus into species and varieties, the 
wave-movements grow more complex that are com- 
municated from the common parent-form to the whole 
group of its descendants ; and every ripple makes its 
individual impression in special manner on its different 

Now, our biogenetic law teaches us that this large 
process of development in the stem history is reflected 
in miniature in the embryonic history of every indi- 
vidual. Here it is the life histories of the constituent 
plastids that correspond to the single waves. The 
cytula, or the first product of the fertilised ovum, 
whence the multicellular organism is developed, bears 
the same relation to the different cell-generations that 
arise from it by fission, and later on form the various 
tissues by specialisation, as the parent form of a class 
or order to the many families, genera, and species that 
arise from it, and develop in varying wise under 
adaptation to varying life-conditions. The ontogenetic 
cell-ancestor of the former has exactly the same form 
as the phylogenetic species-ancestor of the latter. 
The transmitted motion of development that in one 
case passes from the parent-species of the whole group 
of species, in the other case from the parent-cell of the 
whole group of cells, assumes in each instance the 
form of a complex wave-motion. He who understands 


the great biogenetic law will regard it as but natural 
that the microcosm of the ontogenetic genealogical 
tree of the cell represents the diminished and partially 
distorted image of the macrocosm of the phylogenetic 
genealogical tree of the species. 

As we are only able to understand and explain every 
compound and complex phenomenon by breaking it 
up into its individual parts, and by making a most close 
and searching analysis of these, it is essential that we 
work out our mechanical theory of Evolution to its 
final and elementary details. Now, the whole bio- 
genetic process is the very complex resultant of the 
developmental components from all organised species, 
These again are made up of the development processes 
of persons, as these are of those of the constituent 
plastids. But the development of each individual 
plastid again is but the product of the active move- 
ments of its constituent plastidules. Now it has been 
seen that the development movement of phyla and 
classes, of orders and families, of genera and species, 
of persons and plastids, has at all times and places the 
characteristic fundamental form of the complex undu- 
latory movement. Therefore the molecular motion of 
the plastidule, lying at the root of all these events, can 
actually possess no other form. We are bound to 
conclude that this primal cause of life-processes, this 
invisible motion of the plastidule also is a complex 
undulatory movement. We name this true, this effi- 
cient cause of the biogenetic process by the name 
Perigenesis, the rhythmical wave-propagation of the 
living particles or plastidules. In truth, this mecha- 
nical hypothesis is well fitted to explain clearly this 
process to us. 

Whilst then we regard an unbroken, complex wave- 
motion of the plastidules as the working cause of the 
biogenetic process, we see the possibility of tracing 
back the ceaseless and complex phenomena presented 
in the latter, to the mechanical motion of masses of 
atoms. These would be as much conditioned by 
chemical and physical laws as in all the phenomena of 
inorganic nature. If we name this complex wave- 
motion of the plastidules, as perigenesis or wave-propa- 



gation, we shall thus indicate the characteristic pecu- 
liarity which distinguishes this as a complex form of 
motion from all kindred rhythmical processes. This 
peculiarity depends on the power of reproduction of 
the plastidules, and this, in its turn, is conditioned by 
their peculiar atomic composition. That power of re- 
production which alone makes possible the reproduc- 
tion of the plastids, is but the equivalent of the memory 
of the plastidules. And here, once again, we come 
back to the well-founded idea of Ewald Hering, adopted 
already in this essay, that unconscious memory is the 
most important characteristic of organised material, or 
more accurately of the organising plastidules. Memory 
is a large factor in the biogenetic process. Through 
the memory of the plastidules, the plasm is able to 
transmit in continuous rhythmical movements its 
characteristic properties from generation to generation 
by heredity, and to implant in these the new experi- 
ences which the plastidules have acquired by adapta- 
tion in the course of their evolution. 

I have already proved, in detail, in the " Generelle 
Morphologie," that the changes of organic forms that 
we include under the idea of adaptation in its widest 
sense, are conditioned by changed relations in the nu- 
trition of the plastids. But these latter are referable to 
chemical changes in the atomic composition, and con- 
sequently, in the molecular motion of the plastidules. 
These are, by the extraordinary mobility and complex 
arrangement of their constituent atoms, at once affected 
by the changing influence of the surrounding external 
world, or by the external conditions of existence. The 
plastidules do not forget these experiences. They trans- 
mit them to their descendants as modifications of the 
original plastidular motion. Thus heredity is explained, 
as essentially that transmission of individual plastidular 
motions which is essentially bound up with every pro- 
cess of reproduction. 

In the " Generelle Morphologie " (vol. L, p. 154 ; vol. 
ii., p. 297), and in my " Natiirliche Schopfungs-ges- 
chichte (6th edition, p. 226, 300), I had already derived 
every separate organic form as an essential product of 
two mechanical factors, that may be named after the 


fashion of our older biology, constructive forces, or 
stimuli. The internal force, the formative force within 
(named by Goethe the centripetal stimulus or that of 
specification) is heredity. The external force (named 
by Goethe the centrifugal or metamorphic stimulus), is 
the power of adaptation or variability. The latter is 
conditioned by that which Baer called the " degree of 
perfection " — the latter by that which he named " the 
type of formation." Keeping in view our idea of peri- 
genesis, we can now mark off with greater precision 
the opposition between these two fundamental forces 
that determine the structure of organisms, by saying 
that heredity is the memory of the plastidules, varia- 
bility their power of comprehension. The former 
brings about the stability, the latter the variety of 
organic forms. In very simple and very stable forms 
the plastidules have, in a sense, learnt nothing and for- 
gotten nothing. In very complex and variable forms 
the plastidules have learnt much and forgotten much. 
As an example of the former, I quote the embryology 
of Amphioxus, of the last, that of man (" Anthropo- 
genic," viii. and xiv. lectures). 

The differences between my hypothesis of perigenesis 
and Darwin's of pangenesis are evident. Just as Dar- 
win's gemmules or living particles differ essentially 
from plastidules or living molecules, the molecular 
motions that our two hypotheses postulate are entirely 
different. The gemmules of pangenesis are groups of 
molecules that " are able to grow, nourish themselves, 
and multiply by fission, like cells." The plastidules of 
perigenesis, on the other hand are single molecules, and 
as such, do not possess all the properties just enume- 
rated. They are only capable of transmitting their 
individual plastidular motion to neighboring plasti- 
dules, and of forming by assimilation in immediate 
contact with themselves new plastidules of like nature 
to their own, after the fashion of a crystal growing in 
its mother liquid. Further, they are capable of chang- 
ing with great readiness their atomic constitution under 
external influences, and of changing pari passu with 
that atomic constitution, their plastidular motion. Dar- 
win holds that every cell gives off particles to all 


regions of the body, and that all reproductive cells, both 
the egg and sperm-cells, that are concerned in sexual 
reproduction, and the indifferent cells that have to do 
with asexual reproduction, receive gemmules that have 
been given off from all the cells of the organism ; and 
not only of this organism, but of all its ancestors. I 
am unable to see how these could arrange themselves 
in the reproductive cells and form the new organism. 
Indeed, a theory of development on this basis seems to 
me incompatible with the cell-theory, the plastid theory, 
and with our knowledge as to the successive differen- 
tiation and specialisation of cells in the course of onto- 
geny. The specialisation and definite succession seen 
in cell-life, on which I lay the greatest stress, and the 
regular periodicity of the movements of the plastidules 
which from time to time repeat this acquired process 
of specialisation, and render it yet more complex by 
further acquisitions, these have no place in the theory 
of pangenesis. 

On the other hand, my hypothesis of the perigenesis 
of the plastidule is based on the mechanical principle 
of transmitted motion, that which Aristotle looked 
upon as the most important cause of individual de- 
velopment. This great thinker held that in sexual 
reproduction the initiation of the stimulus, or the ex- 
citation to the movements of development, originate 
from the male cells and pass thence to the female cells. 
Further, he combats expressly the idea involved in 
pangenesis that the reproductive cells come from all 
parts of the body. Our plastidules are the constituent 
molecules of the plasm, which the plastid-theory, an 
extension of the protoplasm-theory, regards as the sole 
active factors of plastidular life, whilst only a passive 
role is assigned to the other tissue-molecules by this 
same theory. When the oscillatory molecular motion 
of these plastidules (the plastidular motion) is trans- 
mitted in the multiplication of plastids to the new- 
formed plastids as heredity, it becomes a complex 
undulatory movement. When in the different de- 
scendants the varying conditions of existence exert 
direct influence upon the different branches, new forms 
arise by adaptation. Through the inheritance of these 


adaptations arises in the later descendants the divergent 
specialisation of the plastids, that we regard as the chief 
cause of further evolution. Hence are the wave-circles 
of this ramifying undulation ever more numerous, 
more varying in nature, more complex, the further we 
trace the advancing perigenesis of the plastidules. 

All the manifold, complex, remarkable phenomena 
of the biogenetic process seem to me, in the light of 
perigenesis, capable of a simple mechanical explana- 
tion from a single point of view. On the other hand, 
I have tried in vain to build up a like simple mecha- 
nical explanation by aid of pangenesis, called by Dar- 
win himself a very complex hypothesis. Moreover, all 
the chief phenomena of development, which he tried 
to explain by aid of the hypothesis of pangenesis — 
reproduction and heredity, nutrition and adaptation, 
reversion, alternation of generations, hybridism, re- 
growth of parts, seem to me to find in the pangenesis 
of the gemmules no mechanical explanation recon- 
cilable with the facts of cell-life and of embryonic 
development. But such an explanation is given by the 
perigenesis of the plastidules. Darwin says explicitly : 
" that all forms of reproduction depend on the aggrega- 
tion of gemmules, given off from all regions of the 
body." We say, on the other hand : " that all forms of 
reproduction depend on the transmission of plastidular 
motion, which is directly transmitted from the repro- 
ducing part of the body to the reproduced plastids, 
but further, may reproduce in whole or in part in 
the offspring the undulations of memory and of 
specialisation of the plastidules possessed by their 

That which I have here urged in objection to the 
ingenious pangenesis theory of Darwin, holds to some 
extent of that acute development theory, which, in 1874 r 
Elsberg, of New York, put forward as the theory of the 
regeneration or the preservation of organic molecules 
(Proceedings of the American Association, Hartford, 
1874). In this theory, however, in agreement with our 
plastid-hypothesis, plastidules take the place of gem- 
mules. In his conception of the plastidules as real, 
active, plasm-molecules, and in his notion as to the 


fundamental importance of plasm itself, Elsberg is in 
essential agreement with myself. But he introduces 
the fundamental principle of pangenesis into his theory 
of regeneration. This last he formulates in the follow- 
ing words : " The germ of every reproduced living 
being contains plastidules from all its ancestors. I 
name this the regeneration hypothesis, because, accord- 
ing to it, the ancestors are born again in their offspring, 
to a certain extent bodily, and, indeed, in all other 
ways ; or the hypothesis of the preservation of organic 
molecules, because it holds that certain plastidules are, 
for a long time at least, if not for ever, preserved and 
transmitted from generation to generation ; or I could 
call it the hypothesis of the preservation of organic 
forces, which expresses the same meaning as the 
phrases already given." 

As is evident in this passage and in other passages 
in Elsberg, he agrees on the most essential point with 
the pangenesis hypothesis of Darwin, in that the 
former thinker, as the latter, affirms the material trans- 
mission of actual molecules through the whole series of 
related generations, and with that the material compo- 
sition of every germ out of the corporeal particles of 
all its ancestors. But our hypothesis of perigenesis is 
in direct opposition to this. For we affirm a direct 
transmission of corporeal molecules only from the 
individual that reproduces to that which is reproduced, 
but not from the older ancestors to that list. From the 
ancestors only the peculiar form of the periodic move- 
ment is transmitted or inherited, and this continuous 
wave-motion of the plastidules alone is it that can cause 
the properties of ancient ancestors to appear again in 
their later descendants like a memory. This is the 
very character of advancing undulation, that the kind 
of waves can reproduce themselves from the point of 
origin of the motion over wide areas and in number- 
less parts of the vibrating mass, although the moving 
molecules only move to and fro within very narrow 
limits, within a single wave-length, and the wave itself 
never shifts its place. In a very meaningful and 
significant manner, therefore, we call the undulatory 
motion a reproduction of waves. Adopting this form 



of speech, we may call the reproduction of organisms 
also a special undulatory motion. 

Besides this point of difference, Elsberg seems to me to 
go too far also, in that he thinks the cell theory conquered 
by the histological researches of Beale and Heitzmann, 
and regards the reticulate arrangement of the series of 
plastidules in the plasm or formative matter as a general 
and essential property of all plastids. I regard this 
reticulate arrangement of the series of plastidules in 
the interplastidular substance as a secondary phenome- 
non, and hold that originally (e.g. in the simplest 
Monera) the plastidules alone, placed closely together, 
form the whole body of the plastid. Later, in conse- 
quence of their greater formative capacity, they separate, 
are intercalated among interplastidular-masses, and take 
on the reticulate arrangement that we see so general 
(though not universal by any means) in cytods and 
cells. None the less, Elsberg is in the right when he 
dwells upon the high importance of the plastidules, 
and looks upon them as the essentially active factors in 
the life-process. 

The great group of facts on which we base our hypo- 
thesis of perigenesis, have, for the most part, long been 
known as empirical foundations of the evolution theory, 
and the theories based on them that we have connected 
together by the idea of perigenesis are at present 
accepted by the majority of biologists. We need waste 
no words in support of the cell-theory, with which we 
started. In like manner, of late, general acceptance 
has been given to the idea that the active, formative 
living matter of the cells, the material basis of life 
functions, is to be sought in protoplasm and the allied 
matter of the nucleus, and that all other parts of the 
tissues represent passive constituents formed out of 
these two. The Monera and the ontogenetic embryo- 
form of the Monerula show us that protoplasm and 
nucleus have arisen later on by differentiation from 
simple plasm. Resting on these fundamentals, we 
venture to think that we have shown in our plastid- 
theory that all the numberless kinds of protoplasm and 
of nucleus are only modifications of a single, funda- 
mental, formative matter, plasm itself, and that in con- 


sequence the plasm-molecules or plastidules must be 
regarded as the essential, molecular factors of the bio- 
genetic process. The latter we are bound to describe 
as a peculiar molecular motion conditioned by their 
atomic constitution. It has been generally conceded 
that the biogenetic process as a whole, as well as in par- 
ticular parts, may be represented as a complex undu- 
latory motion. As we can find the effective cause of 
this very complex undulatory motion in the molecular 
motion of the plastidules alone, we are bound to speak 
of this latter motion also as an undulation. 

If we wish to make out on rigidly mechanical 
grounds a claim on behalf of our hypothesis of peri- 
genesis to the value of a theory of evolution, we should 
lay especial emphasis on the nature of the rhythmical 
molar-motion, the complex wave-movements that the 
biogenetic process incontestably presents. Then there 
only remains as a hypothetical element in the theory, 
the sum of the properties ascribed to the plastidules or 
plasm-molecules. We regard these particles of living 
matter as the true active factors of life processes, and 
attribute to them, in addition to those properties com- 
mon to all molecules or little masses compounded of 
atoms, a peculiar property which distinguishes them as 
living molecules from others. This property, which 
alone really distinguishes the living organism from the 
non-living inorganic body, is the faculty of memory or 
reproduction. Without this hypothesis the manifold 
phaenomena of reproduction and development seem 
absolutely incomprehensible. Ewald Hering, in the 
work already quoted more than once, has clearly shown 
on what good grounds the idea of such an unconscious 
memory of the plastidule is based. On the other hand, 
it has not been possible for me, after careful considera- 
tion, to find any tenable objection to this hypothesis. 
Therefore I consider memory, or the power of repro- 
duction of the plastids, as a function of plasm, condi- 
tioned directly by the atomic composition of the plasti- 
dules themselves. 

From this point of view we may, perhaps, call 
perigenesis a mechanical theory in a wider sense, or, at 
least, a hypothesis that may serve as the germ of such 


a theory. Another thing that speaks greatly in its 
favor, is its great simplicity. This is, as a rule, evi- 
dence of a theory in harmony with fact. How simple 
are the principles of Newton's theory of gravitation, 
Huyghen's undulatory theory, Myers' of temperature, 
Schleiden's cellular theory, that of descent of Lamarck, 
that of selection of Darwin, Yet by these simple prin- 
ciples the greatest and most extensive collections of 
different facts are connected into one, are explicable as 
due to a common cause. Just as simple is the principle 
of complex undulatory motion of the plastidules con- 
sidered by us as the effective, mechanical cause of the 
biogenetic process. 

The Monistic science of the present day rightly 
enough lays claim to explain all natural phenomena 
mechanically, and to trace them all back, teleology 
excepted, to efficient causes. This first duty is satisfied 
by our theory of perigenesis. For the principles of 
transmitted molar motion and the conservation of 
energy that lie at its base are purely mechanical. 
Purely mechanical also is the principle of autogony, 
that derives the first stimulus to this transmitted 
motion from those movements of atoms that occur in 
the formation of the first plastidules, and are effected 
by the peculiar motion of the plastidules. We have 
been able to refer heredity to the transmission of these 
plastidular motions, adaptation to their changes, and 
these are the two chief factors in the building up of 
organic forms. Thus the biogenetic process is in un- 
constrained harmony with the law-abiding march of 
all the processes in the universe-. It is a special and 
very complex form of rhythmical molar motion. And 
its effective cause is the perigenesis of the plastidules. 




THE fertile influence exercised on the most different 
branches of science, and especially on natural his- 
tory, during the last eighteen years by the new study 
of development has worked on none more swiftly, has 
brought forth rich fruit on none more than on the 
domain of organic morphology, the study of the forms 
of animals and of plants. In the first place, as a con- 
sequence of the new theory of descent, various im- 
portant branches of inquiry, hitherto separated to a 
greater or less extent one from the other, are found to 
be closely allied, to be in the most intimate connexion 
and mutual relation. The consideration of internal 
and of external forms, comparative anatomy and classi- 
fication, embryology and paleontology, are recognised 
in the clear light of the theory of descent as intimately 
connected sciences, moving by various routes to one 
and the same end, the knowledge of organic forms 
through that of their historic origin. As result of this 
a new science has evolved, whose immediate aim is the 
knowledge of this origin of allied animals and plants ii 
genealogical connexion, and the discovery in the genea- 
logical tree of these forms of a true natural system. 
This new science is ancestral history or phylogeny. 

Every new science has to contend, at first, with the ill- 
favor and jealousy of its elder sisters, who fear from it 
injury to their older well- won rights. This fear is the 
greater the higher are the aims of the new comer, the 
wider the area of work it seeks to win. This is of 
value in that thus its young strength is tried in the 
hard struggle for existence, and like a young plant in a 
thickly populated field it has to wrestle with envious 
sisters for room, for light, for air. Thus the youngest 
of sciences and not the least hopeful, comparative 



philology, has to gain its own strength in the heated con- 
test with other branches of philology. And phylogeny, 
whose aims and methods are closely allied to those of 
comparative philology, has not been spared that neces- 
sary struggle for existence. 

When we made the first attempt, ten years ago, in 
the " Generelle Morphologie," to lay down the concep- 
tion and ends of phylogeny, that attempt met almost 
everywhere with suspicion and derision, with scorn 
and hostility. How will this pretentious phylogeny 
reveal the secrets of organic creation ? How will it 
prove the hypothetical genealogical tree of organisms ? 
And what records are at its command in its inquiries 
into prehistoric things ? Such doubts and the like as 
to the possibility, to say nothing of the consequences 
of phylogenetic inquiries, were loud and general, and 
whoever was not very intimate with the subject of 
organic morphology, and with the vast extent of its 
stores of knowledge yet uncoined in words, could not 
but look on our attempts from the first as hopeless,, 
as courting defeat. 

How lies the matter to-day, after a lapse of but ten 
years ? We ought to be quite content with the results 
of this first decade in the history of phylogeny. Our 
foes ought not to grudge us our consciousness of 
decisive victory. Not only has phylogeny attained an 
independent value and a recognition in natural history y 
in biology, not only are phylogenetic ideas and princi- 
ples forming already essential parts of our best treatises 
and handbooks. Many valuable special investigations 
into particular questions in phylogeny have already 
begun, and have, though as yet incomplete, brought to 
light results of the most startling description. Nay, 
at this hour we enjoy this much of triumph. Many 
of our opponents have made complete recantation, and 
are themselves treading the path first entered upon by 
us, and by them regarded as impassable. The ablest 
zoologists and botanists have accepted with one accord 
the phylogenetic method, and by use of it have already 
attained results that without it could never have been. 
The maligned genealogical trees, which phylogenetic 
nvestigation uses as the simplest, clearest, most con- 



densed expression of its inventive hypotheses have 
received unexpectedly swift recognition, and are of 
general use in morphology. It is true that voices are 
not wanting to declare all these phylogenetic efforts as 
so much empty trifling. Only recently, we might hear 
from the mouths of recognised physiologists that our 
" genealogical trees were of the same value as are the 
pedigrees of the Homeric heroes in the eyes of the his- 
torical critic." But these utterances, and others as 
contemptuous, only prove that the physiologists con- 
cerned are wholly ignorant of the present condition of 
morphology, and have no idea of its range and signifi- 
cance. And further, one may read, not without sorrow, 
between the lines, that physiology in its one-sided 
fashion knows not how to make use of the study of 
descent, whilst morphology has obtained by the help 
of that study the greatest results. But just as in- 
effectual as the like ignorant prejudices were against 
comparative anatomy which has stood firm for seventy 
years, or against classification which has done the 
like for twice that time, and has employed thousands 
of busy workers, so will they be against phylogeny, 
at once youngest and most hopeful child of scientific 

Further, the esteem in which phylogeny is held, is 
very different within the limited circle of those skilled 
in morphology and in the wider circle of cultured lay- 
men. Especially is opinion divergent as to the worth 
possessed by the empirical records of phylogeny, and 
the stability of the hypotheses and genealogical trees 
based on these. Hence it seems wise, in this place, to 
cast a glance of inquiry over the records of phylogeny, 
and so ask how far in our building up of phylogenetic 
hypotheses we can rest on substantial facts. It is true 
that we have already made known our ideas on the 
value and significance of the different " evidences of 
creation " in our " Naturliche Schopf ungs-geschichte " 
(Edition vi., Lecture 15), and " Anthropogenie " (Edi- 
tion iii., Lecture 15). But, nevertheless, as since then 
the ideas of other scientists on this question have 
differed greatly, it is not superfluous to bring back 
the over-estimation or under-estimation, on one side or 



the other, of these very important evidences to their 
true value. 

I take the ground that in reality there exists no 
domain of natural history that does not yield us evi- 
dence of greater or less value in favor of phylogeny. 
Not only all branches of morphology, but also different 
branches of physiology, e.g. chorology, the study of the 
geographical and topographical distribution of orga- 
nisms, give us facts that we can utilise directly or 
indirectly on behalf of phylogeny. But before all 
other branches of science three are most prominent as 
the most especially in evidence for phylogeny. These 
are comparative anatomy, ontogeny, palaeontology. 

Palaeontology, or the study of fossils, furnishes at the 
present time in many ways the most reliable and most 
accessible kind of evidence as to the order of creation 
in the past. For the fossils or petrified remains of 
plants and animals that we meet with in the sedi- 
mentary strata of the earth's crust are, in truth, the 
fossil remains or impressions of those organisms, long 
dead, that hundreds of thousands or many millions of 
years before peopled our earth. Amongst these orga- 
nisms also, in conformity with Evolution, must have 
been the veritable ancestors of the species of plants 
and animals in existence to-day, and the allies more or 
less closely related of those dead and gone ancestors. 
Hence many naturalists, especially those who wish to 
proceed as carefully and exactly as is possible, as well 
as those who would extend palaeontology yet further, 
place in that science their greatest hope, and regard it 
as the sole reliable evidence in favor of phylogeny. 

It is at the present time generally known that fos- 
sils are of very great significance and importance as 
actual medallions of creation. They alone are our 
immediate teachers as to the appearance and the 
historic changes of form in the various species of 
animals and plants in that long series of epochs in 
nature that are numbered in millions of years. They 
alone show us plainly what a wealth of differing 
species the individual groups of the animal and vege- 
table kingdoms present in the different sections of the 
earth's strata. They alone enable us to form a general 



picture of the characteristic physiognomy of the organic 
population in the different epochs through which our 
planet has passed. Finally, by the fossils alone are we 
taught how the special ancestral history of particular 
species and genera, the detailed ancestral tree of these 
last, may be traced out step by step, branch after 
branch. Thus we have lately, e.g., been enabled by 
certain startling palaeontological discoveries, to trace 
back, step by step, the pedigree of our modern horse 
to tapir-like forefathers. In like fashion we are able 
to trace out to a great extent, with varying degrees of 
certainty, the ancestral series of our cattle and of our 
swine. The genealogy of many Mollusca with cal- 
careous shells, especially of the ammonites, is also 
known in detail with a degree of certainty that is 

But startling and palpable phylogenetic results such 
as these accruing from palaeontology are, unfortunately, 
very rare exceptions, and in general it may be said 
that the value of palaeontology as phylogenetic evi- 
dence has been greatly over-estimated. For valuable 
and incontrovertible as this most direct and most sure 
evidence is intrinsically on the one hand, on the other 
it loses value on account of its extraordinary uncertainty. 
This results in part from the nature of the organisms, in 
part from that of the rocks in which they have left 
their fossil remnants and impressions, in part from the 
nature of the process of f ossilisation itself. The great 
majority of all organic forms are so soft and delicate, 
or live under such conditions, that rarely or never are 
they able to leave behind them a fossil of any value. 
Hence we obtain by aid of palaeontology nothing, or 
almost nothing, of many classes of animals and plants, 
and of the soft embryos and earlier conditions of all 
organisms. But the hard and solid parts, which alone 
are capable of preservation as fossils, the skeletons, are 
in the different groups of animals of very varying 
value. Hence, e.g., the fossil remains of Vertebrata, 
Mollusca, and Radiata are full of instruction, whilst 
the fossil remains and impressions of most insects, 
worms, and zoophytes (corals excepted), are of very 
limited significance. 



In addition to this great deficiency in palaeontologi- 
cal evidence, there is the difficulty that all the older 
sedimentary rocks, all the formations laid down before 
the Silurian and Cambrian age, have been, under the 
action of heated liquids from the interior of the earth, 
changed or metamorphosed in whole or in part, so that 
they contain only very few or absolutely no recognis- 
able fossils. Hence of all the strata that belong to the 
Laurentian period, that stupendously long period in 
history, in which the organic world began to evolve 
and to advance towards the differentiation of the chief 
large groups of the animal and vegetable kingdoms, we 
must expect, as a rule, no evidence in the shape of 
fossils. And to this rale such Laurentian fossils as the 
much discussed Eozoon, so full of significance, are, 
alas, rare exceptions. Moreover, in many other forma- 
tions, that contain many petrefactions, these occur in 
such bad condition and unrecognisable form, that they 
are of no value in phylogeny. 

These and other circumstances, that are based upon 
the nature of organisms, of processes of fossilisation, 
and even on conditions of rock formation diminish to 
an extraordinary extent the significance of palseonto- 
logical evidence, and compel us to conclude that of the 
vast majority of organic species that have lived on our 
globe, we shall never learn anything through the 
medium of their fossil remains. Of course, as yet, only 
the greater part of Europe and of North America has 
been carefully examined in regard to fossils. The rest 
of the regions of earth are, for the most part, still un- 
explored. We hope that palseontological investigation of 
them will make us acquainted with very many and very 
important fossil remains. But under no circumstances 
will these same be able to fill up all the gaps that 
exist, and to demonstrate beyond controversy the whole 
ancestral history of organisms by an unbroken series of 
fossils. Hence we are in need of other, more convinc- 
ing evidence, and this we find partly in comparative 
anatomy, partly in ontogeny. 

The comparative anatomy of animals and plants 
recognises certain characteristic relationships in the in- 
ternal structure of those organisms, especially in the 



relative disposition and arrangement of the organs, that 
are common to all the members of a natural group or 
" type," despite the greatest diversity of external form. 
The number of these main groups or types in both the 
animal and vegetable kingdom is very small. In the 
latter we distinguish, as a rule, only three or four, in 
the former six to eight. Only within each type is a 
rigid, morphological comparison of all the bodily or- 
gans possible. Only within each type can we speak of 
true relationship of form. This internal and essential 
community of bodily structure, standing in remarkable 
contrast to the variety of external forms, was explained 
in the older comparative anatomy by the mystic idea 
of a " unity of design " or plan of creation. Since the 
reformation of the study of development, we explain it 
very simply and naturally as the result of the common 
origin from a parent form. This parent form trans- 
mitted all essential characteristics of its internal struc- 
ture by heredity, with more or less completeness, to all 
its descendants, whilst these acquired by continued 
adaptation the most various differences in external 
structure and in non-essential particulars. Every type 
becomes thus a stem or phylum. The typical relation- 
ship of form becomes the real relationship through an- 
cestry, conditioned by heredity. The aim of compara- 
tive anatomy is to distinguish and demonstrate the true 
kinship of the different as well as to explain the like- 
ness of related form by aid of heredity from common 
ancestors, and adaptation to similar life-conditions. 
Comparative morphology is accordingly broken up into 
homology and analogy. Homologues are like organs 
that have originated from one and the same common 
ancestral form by adaptation to different functions. 
Analogues are like organs that have arisen from differ- 
ent ancestral forms by adaptation to similar functions. 
The pectoral fins of fishes, the wings of birds, the fore- 
limbs of quadrupeds, the arms of man, are homologues. 
The wings of birds and of insects, the fins of fishes, the 
swimmerets of the lobster, the " wings " of pteropods, 
the legs of quadrupeds, the legs of insects, are an- 

It has long been known, that within every type or 



phylum (e.g., within that of the Vertebrata) long series 
of forms occur, ranging from lower to higher, from the 
imperfect to the perfect, from the simple to the com- 
plex. Consider the long series of advancing develop- 
ment in all organs from the lowest vertebrate to the 
highest, from Amphioxus to man. But these series are 
not simple, ladder-like, but complex, tree-like, since 
starting from the original, simple, common forms, ad- 
vancing improvement is carried out in different direc- 
tions and in various wise. This tree-like arrangement 
of related forms, that the system of animal and vege- 
table classification acquires under the directing hand 
of comparative anatomy, expresses the facts of Evolu- 
tion in a manner more actual than the genealogical 
tree. It is clear that this same pedigree, that repre- 
sents the natural system of organisms, cannot be laid 
down with absolute certainty, but only approximately. 
But this is a necessary result of the nature of the ques- 
tion at issue, and in no sense lessens the value of the 
work that is done. 

The ideas of different morphologists, nowadays, 
differ widely as to the value of comparative anatomy in 
the construction of a natural system, and as to how far 
we have the right to assume that such a natural system 
is a veritable genealogical tree. Some ascribe to it the 
highest, others the lowest significance, whilst yet 
others, holding a middle course, would mete out to it 
an average measure of importance. In reality, all this 
depends upon the different capabilities and power of 
comprehension of the morphologists concerned. Narrow 
minds, short-sighted observers, confining themselves al- 
ways to the most evident and recognisable facts, are 
incapable of taking in at a glance large groups of re- 
lated phenomena of form, after the fashion of compara- 
tive anatomy. Further, they are unable to distinguish 
between the essential and the non-essential, that which 
is important, and that which is accidental. Narrow, 
small minds of this order, although adapted by their 
very smallness and narrowness to become excellent 
specialists and species-mongers in science, can never 
estimate the value of comparative anatomy, and in large 
or small measure they reject its phylogenetic applica- 



tion. On the other hand, its value has been estimated 
to the full by philosophical thinkers, and by those large 
natures that are capable of taking in at a glance the 
whole of the vast domain of phenomena, and thus are 
able to distinguish the essential from the accidental. 
Such men as these will look upon comparative anatomy 
as the most important of all evidences in ancestral 
history, and will assign to it the first place in the 
building up of a natural system. 

But this evidence as to the past, valuable as it un- 
doubtedly is, has its weakness. And this is based, in 
the first place, upon the imperfection of its materials, 
and in the second place, upon the difficulty of distin- 
guishing in all cases clearly between homology and 
analogy. A great number of connecting links between 
living forms in existence at the present day have 
perished long ago, and we are obliged to fill up the 
gaps that result by guesses. Many anatomical relations 
are so complex, that the phylogenetic explanation of 
them becomes, as a rule, exceedingly difficult. Just 
in proportion, therefore, as we estimate comparative 
anatomy at its true value as an important evidence in 
regard to descent, and as we are of opinion that it may 
become altogether over-looked, in the same degree 
must we warn ourselves, on the other hand, against an 
application of it that is too exclusive and one-sided. 
And when, of late, it was stated that in all phylogenetic 
questions the first and last word belonged to compara- 
tive anatomy, we were, and are, unable to share that 
opinion. We are rather of opinion that in many ques- 
tions, and certainly in many of those that are most im- 
portant, ontogeny, the third of our three chief witnesses, 
is of higher significance, of more decisive worth. 

Ontogeny, or the history of the embryo, as we name 
in brief the history of the development of the individual, 
is at the present time very often undervalued in respect 
to its use as evidence in regard to the past, to the same 
extent as palaeontology is over-estimated. We actually 
see the singular spectacle of many embryologists, 
specialists who have made the study of the develop- 
ment of the germ their chief occupation, denying to 
that study any phylogenetic value. Nevertheless, he 



who approaches this branch of knowledge with the 
understanding mind, who does not content himself 
with the mere observation, interesting as that is, of 
ontogenetic facts, but inquires into their phylogenetic 
causes, such a one will certainly have the conviction 
that ontogeny ranks among the most important and 
meaningful evidences as to the history of ancestral 
forms. Here, as in comparative anatomy, it is indis- 
pensable to carry on empirical investigations in a phi- 
losophical spirit, and to search amongst the infinite 
world of phenomena for those general facts which are 
common to the manifold forms that development as- 
sumes. In this study, as in that, it is especially neces- 
sary clearly and sharply to mark off the essential from 
the non-essential, that which is of moment from that 
which is an accidental accessory. 

The phylogenetic importance of ontogeny — the value 
of the study of the embryo as an evidence in regard to 
pedigree — is, in the first place, based upon the fact that 
every organism, in its development from the egg, runs 
through a series of forms, through which, in like suc- 
cession, its ancestors have passed in the long course of 
earth's history. The history of the embryo, therefore, 
is a picture in little an outline of that of the race. 
This conception forms the gist of our fundamental 
biogenetic law, which we are obliged to place at the 
head of the study of development, as the veritable 
fundamental law of organic development. We regard 
it as indispensable, as the chief principle that serves us 
in explanation of that organic development. Every 
advance in ancestral history that our forefathers made 
in their adaptation to new conditions of life, and that 
brought into being some new ancestral form, is repeated 
to-day by virtue of heredity in that history of the 
individual that corresponds with the history of the 
race. Just as at this hour every organic individual 
takes origin from a simple ovum-cell, so the common 
ancestors of all the species of any phylum was originally 
a simple cell. 

Now, it is evident that only in rare cases, only in a few 
lower organisms, is this recapitulation of the ancestral 
history that we see with our own eyes in the embryonic 



history quite complete. In the great majority of cases 
the recapitulation is exceedingly abbreviated. Often, 
moreover, it is modified, and very frequently quite meta- 
morphosed. This results from the fact that the young 
embryo, from the very outset of development, is sub- 
ject to the transforming influence of external life- 
conditions, and adapts itself to them. As result of 
these " embryonic adaptations," entirely new formative 
elements enter into the individual course of develop- 
ment that, to a greater or less degree, alter the original 
process of development. In especial, a condensation of 
the original repetition of events occurs very frequently 
— the more frequently the higher the stage of evolution 
reached by the particular organism — whilst some or 
many stages in development disappear. At other times, 
on the contrary, quite new links are intercalated in the 
inherited chain of forms. We may denote all these 
later modifications of the original palingenetic process 
of development — in a word, all " bastard or spurious 
modifications " — as cenogenetic (kci/os:= unimportant). 

Hence, all phenomena that we observe in the course 
of the development of the individual animal or plant, 
from the ovum-cell up to the complete formation of the 
adult body, fall into two great groups, the palingenetic 
facts or those of condensed history, and the cenogenetic 
facts or those that are spurious, and tell us nothing as 
to the history of the past. The ontogenetic facts of 
palingenesis alone are of direct value as evidence in 
respect to pedigree, and they alone have relation to 
corresponding precedent changes in phylogeny. On 
the other hand, the ontogenetic phenomena of ceno- 
genesis have not only no kindred phylogenetic meaning, 
but are, on the contrary, will-o'-the-wisps. We must 
take heed that we are not misled by their false 
glimmer. The fundamental biogenetic law must there- 
fore now be more rigidly formulated, as follows : " The 
history of the embryo is an epitome of that of the race ; 
the more complete, the more the condensed develop- 
ment has been retained by virtue of heredity ; the less 
complete, the more a spurious, non-historic development 
has supervened by virtue of adaptation." In my " An- 
thropogenic " I have tried to prove, by taking man as 



an example, how the fundamental law of organic deve- 
lopment, thus formulated, finds its application, and 
how by aid of it we can, from the actually observed 
facts of embryology, draw the most important conclu- 
sions as to the hypothetical precurrent events that 
occurred in the history of the race. 

If now, therefore, we regard ontogeny or embryology 
as the most important and essential of all the evidences 
as to phylogeny, we shall still, in doing thus, by no means 
lessen the great value that the other kinds of evidence, 
and ©specially comparative anatomy, possess. Without 
aid of this last we should not be in a position to under- 
stand the phenomena of embryology nearly so clearly, 
nor to estimate them with nearly so much accuracy, as we 
can, in fact, now understand and estimate them. Com- 
parative anatomy and ontogeny are in the happiest 
fashion complementary one to the other, and in recip- 
rocal manner each fills up the gaps of each. If, there- 
fore, of late certain morphologists have regarded com- 
parative anatomy exclusively, and others comparative 
embryology exclusively, as the only reliable witness on 
the points at issue, we are bound to look upon the 
points of view of both sets of thinkers as one-sided and 
deficient. We shall only be able to know the ances- 
tral history of organisms by complete and conjoint in- 
vestigation of both these two chief sets of evidence on 
the question. It is clear that it is of the first importance 
to this end that man should be intimately acquainted 
alike with the rich empirical treasures of both sciences, 
and that is certainly not the case with these lop-sided 

This much at the present time is certain beyond a 
doubt, that for the construction of a phylogenetic 
history an exceedingly rich storehouse of empirical 
evidence, of certain facts is at our disposal, that only 
needs to be taken possession of and understood in order 
to be recognised in all its full significance. It is not in 
this connexion a question of discovering new and un- 
known facts bearing upon the pedigree of organisms — 
and also on that of man — but of understanding and utilis- 
ing the facts already to hand. Sources more rich, more 
full of meaning, than comparative anatomy and onto- 



geny will never be discovered. With their help alone 
we are already in a position to found the new science 
of phylogeny, even if we ignore altogether those less 
important sources of information accruing to us from 
palaeontology, chorology, and other auxiliary sciences. 
But if many men — and among them even some scientists 
of repute — hold that the whole of phylogeny is a castle 
in the air, and genealogical trees are empty plays of 
phantasy, they only in speaking thus demonstrate their 
ignorance of that wealth of empirical sources of know- 
ledge to which reference has been already made. 

The aims and ways of phylogeny are the same as 
those of geology. Just as the hypothetical history of 
evolution of the earth is based upon a scientific founda- 
tion, as firm as it is beautiful, on the ground of experi- 
mental evidence, so is it also with its younger sister, 
the ancestral history of organisms. The one can, and 
will, rise to the position of a real "exact" science as 
little as the other. For the historical events of past 
time, whose connexion each of these branches of know- 
ledge aims at demonstrating, occurred many millions 
of years ago, and are for ever removed from direct 
observation on our part. Hence geology and phylogeny 
alike are from the nature of things historical sciences. 
But the foundations of the hypotheses that belong to 
the latter, as of those that appertain to the former, repose 
upon a world of the securest proofs. And just as 
the value of the geological evidence is to-day generally 
recognised and utilised in the history of the evolution 
of the globe, so day by day grows the recognition of the 
incalculable worth that our morphological evidence has 
in regard to the ancestral history of organisms. 



T 2 


ON this festal day that gathers us together in this 
place at the opening of the fiftieth meeting of 
German scientific men, the relationship of science as a 
whole to our especial branches of enquiry has an 
importance greater than all other questions. Whilst 
cultured men of all classes trace out the astounding 
advances of natural science with keenest interest, they 
ought on such a day as this, with special emphasis, to 
ask what general results natural science has yielded in 
the domain of human cultivation. If, therefore, I, in 
obedience to the summons that you have given me — 
a summons that carries with it so much of honor — claim 
your close attention for a little space, I am thinking 
that I can choose no subject more worthy of our 
common consideration than the relation of knowledge 
as a whole to that branch of scientific enquiry that 
comes most home to me personally — the study of 

For more than ten years past no other study has 
taken so strong a hold on general attention, none other 
has entered so deeply into our most important con- 
victions, as the new science of Evolution and that 
Monistic philosophy that is so intimately knit up 
therewith. For by it alone is the question of questions, 
the foundation question as to man's place in Nature, 
to be solved. As man is the measure of all things, it is 
evident that the ultimate questions and highest prin- 
ciples of all knowledge must depend upon the place 
that our ever-advancing knowledge of Nature assigns 
to man in Natupe. 

To Charles Darwin especially our science of Evolu- 
tion to-day owes this — its lordly place in the front 


rank. 1 He it was who, eighteen years ago, first broke 
through the rigid icy barriers of the reigning prejudice, 
and brought into life a unity of development of the 
universe on the same fundamental principles as those 
which our greatest poets and thinkers, a century before, 
had initiated. At the head of these were Immanuel 
Kant and Wolfgang Goethe. 2 By the enunciation of 
his theory of Selection, of the idea of Natural Selection 
in the struggle for existence, Darwin was enabled to 
lay the firm foundation of the biological part, and that 
the most important part, of that general theory of 
Evolution that, in the beginning of our century, saw 
the light as the descent theory. Up to that time the 
older naturalists had fought in vain on behalf of this 
latter theory. Neither Lamarck 3 and Geoffroy St. Hilaire 
in France, nor Oken 4 and Schelling in Germany, were 
able to carry it to victory. It is now exactly fifty 
years since Lorenz Oken began here, in Munich, his 
academic lectures on the theory of development. 
Hence it seems good to us in this place, and at this 
hour to place upon the grave of this zoologist so far- 
seeing, this philosopher so gifted, a laurel-crown. For 
Oken it was, who in 1822, inaugurated at Jena the first 
gathering of German naturalists for scientific purposes. 
To him, therefore, the thanks of this, the fiftieth of 
such assemblies are especially due. 

But the natural philosophy of that time was but able 
to sketch a general plan, a preliminary outline of the 
mighty edifice that we call to-day the theory of Evolu- 
tion. The building stones of that edifice were gathered 
together by the assiduous labors of many men during 
the succeeding half -century. A voluminous literature, 
a marvellous improvement in the methods of scientific 
inquiry, bear very striking witness to the astounding 
advances made in science during that period. But it 
is certain that the measureless expansion of the field 
of empirical observation, and the specialisation that 
resulted thence, often led to a pernicious diffusion of 
energy. The higher aim, the knowledge of general 
laws, was almost forgotten in the interest in the ob- 
servation of particular details which are of secondary 


Hence it could come to pass, that during the period 
of this rigidly empirical inquiry into Nature, from 
1830 — 1859, nearly thirty years, the two chief divisions 
of natural history were working upon absolutely antago- 
nistic bases. In the history of the development of the 
earth, since 1830, since the appearance of Lyell's " Prin- 
ciples of Geology," the general conviction that our 
planet had originated by a supernatural act of creation, 
or had arrived at its present condition through a series 
of complete revolutions of mystic origin, was dispelled. 
It was seen, on the other hand, that an all-powerful 
unbroken evolution had determined its natural forma- 
tion, step by step. But in the history of the develop- 
ment of the living inhabitants of earth, the old unrea- 
soning myth had held general sway, according to which 
the individual species of plants and animals, like man 
himself, appear independently one of the other, and a 
series of such created forms had succeeded one another 
without any genetic connexion. 5 The contradiction 
between the two orders of thought, between the natural 
theory of development of the geologists, and the super- 
natural creation-myth of the biologists was first decided 
by Darwin, in 1859, in favor of the former. Since that 
time, we know of a surety that the formation and the 
variations of form of the living earth-dwellers of our 
globe, follow the same vast, eternal laws of mechanical 
evolution as the globe itself and the whole universe. 

To-day there is no necessity to state the grounds of 
proof on which the modern theory of development, as- 
sociated with the name of Darwin, rests. That was 
done fourteen years ago at the Naturalists' Association 
at Stettin. 6 Since that time the recognition of its truth 
has made general way in very delightful fashion. In 
the particular branch of natural science, in which my 
own work lies, in the wide domain of the study of 
organic forms or morphology, it is already recognised 
by all as the most important basis of our work. Com- 
parative anatomy and embryology, systematic zoology 
and botany, are no longer able to dispense with the 
study of the theory of descent. For, in the light of 
this alone is it possible to explain really the mysterious 
relationships of the innumerable organic forms to one 


another, i.e., to trace them back to mechanical causes. 
Their likeness is a natural result of heredity from com- 
mon ancestors ; their dissimilarity is the necessary 
consequence of adaptation to different life-conditions. 
By the theory of descent alone the facts of paleonto- 
logy? of chorology, of cecology, 7 are explained as simply 
as naturally. By it alone are we able to explain the 
existence of the remarkable rudimentary organs, eyes 
that see not, wings that are never used in flying, mus- 
cles that do not contract, utterly useless parts of the 
body that give the lie to the earlier teleology. 8 For they 
prove in the clearest manner, that adaptation to function 
in the structure of organic forms, is neither general nor 
complete. Such adaptation is not the outcome of a 
designed creative plan, but is brought about of neces- 
sity by the accidental coincidence of mechanical causes. 

He who in the face of these overwhelming facts still 
asks for proofs of the theory of descent, only shows in 
thus asking his want of knowledge or want of judg- 
ment. It is preposterous to demand on its behalf 
direct, experimental proof. This enquiry, so often 
repeated, springs from the wide-spread misconception 
that all natural science must be exact. All the other 
branches of knowledge are often named in contradis- 
tinction to natural science, metaphysics. But in truth 
only the lesser part of natural or physical science is 
exact, that part, namely, which is based on mathe- 
matics : astronomy and higher mechanics generally, 
the majority of physics not included in the higher 
mechanics, most of chemistry, a great part of physio- 
logy, but only a very small part of morphology. 9 The 
phaenomena in this last biological domain are too com- 
plex, too variable for us to make use, as a rule, of 
mathematical methods. And whilst the principle is 
established that we should aim at a foundation for all 
our knowledge as exact as possible, and where possible 
a mathematical foundation, yet as far as concerns the 
greater part of biological science, it is not possible to 
carry out that principle. In this department of 
thought the historical rather than the exact, mathe- 
matical and physical method obtains. 

This holds especially of morphology. For we can 


only acquire a scientific knowledge of organic forms 
through the history of their development. The great 
advance of our age in this domain of knowledge 
consists in this, that we have a far wider conception of 
the end and aim of the study of development than was 
the case until the time of Darwin. For until that time 
we only understood by the phrase, that history of the 
development of the organic individual which we call 
to-day embryology or ontogeny. When the botanist 
traced out the growth of the plant from the seed, the 
zoologist the up-building of the animal from the egg, 
he held that with the completed observation of this 
history of the embryo he had solved his morphological 
problems. The greatest workers in the realm of em- 
bryology, Wolf, Baer, Remak, Schleiden, the whole of 
the school of embryologists founded by these, under- 
stood, in a word, by the study of development exclu- 
sively the embryology of the individual. To-day it is 
far otherwise, for the mysteries of the wonderland of 
embryology are no longer insoluble riddles to us. 
Their deep meaning is now clear. For on the laws of 
heredity the transition forms through which the germ 
passes under our very eyes in a very brief space of 
time are a condensed, an abbreviated repetition of 
corresponding transition forms through which the 
ancestors of the organism in question passed in the 
course of many millions of years. When to-day we 
place a hen's egg in the hatching-machine and see in 
twenty-one days a chick emerge thence, we no longer 
stand smitten dumb with wonder at the marvellous 
changes that lead us on from the simple ovum-cell to 
the Gastrula with its two layers, thence to the vermi- 
form embryo, destitute of skull, and thence to the 
further stages that exhibit in essence the organisation 
of a fish, an amphibian, a reptile, and, finally, a bird. 
Rather is it that we deduce thence the corresponding 
succession of forms of those ancestors which have led 
up from the unicellular Amoeba to the Gastraea, and 
onwards through the classes Vermes, Acrania, Pisces, 
Amphibia, Reptilia to the Aves. The succession of 
embryonic forms in the chick gives us a condensed 
picture of its actual ancestral series. 


Our fundamental biogenetic law formulates the direct, 
causal connexion between the embryonic history of 
the organic individual and the ancestral history of its 
forefathers in this short sentence : the history of the 
embryo is an epitome of that of the race, conditioned 
by the laws of heredity. 10 And this palingenetic epi- 
tome only seems interfered with to any particular 
extent when, by adaptation to the conditions of embryo- 
nal life, cenogenetic alterations have taken place. 11 

This phylogenetic explanation of the ontogenetic 
phaenomena is, up to the present time, the sole expla- 
nation in regard to the latter. But it receives from the 
results of comparative anatomy and paleontology, the 
weightiest confirmation and enlargement. It is evident 
that it does not admit of exact or experimental proof. 
For all these biological studies are of the nature of 
historical and philosophical science. Their common 
aim is the knowledge of historical events that occurred 
in the course of many millions of years, on the surface 
of our planet in its youth, long ere the appearance of 
the human race. Direct, exact knowledge on these 
matters lies quite beyond the domain of possibility. 

An approximate knowledge is only possible as the 
result of a critical use of historical evidence, and of a 
speculation as careful as bold. Phylogeny uses these 
historical evidences in the same way, estimates them 
after the same method, as other historic studies. As 
the historian by aid of chronicles, biographies, letters, 
pictures to us a long past age ; as the archaeologist by 
the study of pictures, inscriptions, hieroglyphics, reveals 
to us the state of civilisation of peoples long perished ; 
as the linguist by comparative investigation of all the 
living languages, and their older memorials in writing, 
that are related to each other, shows to us their origin 
and development from one common primal form ; in 
just the same way the naturalist, to-day, by a critical 
use of phylogenetic evidence, of comparative anatomy, 
ontogeny, palaeontology, has contributed to our ever 
increasing knowledge of those former changes which, 
in the course of measureless ages, befel the forms of 
organic life on our earth. 12 

Phylogeny, or the ancestral history of organisms, 


admits of as little exact or experimental proof as its 
older and more favored sister, geology. The high 
scientific value of the latter has been only quite recently 
generally recognised. The ignorant alone laugh incre- 
dulously, to-day, at the explanation that the huge alpine 
masses, whose snow-clad summits shine out upon us 
from afar, are none other than consolidated sea-mud. 
The structure of these stratified mountains, the nature 
of the fossils that they contain, admit of no other expla- 
nation. None the less, the fact does not admit of exact 
proof. In like manner, almost all geologists agree in 
recognising a definite systematic succession of rocky 
layers, corresponding to different periods of time. And 
yet this system of strata is nowhere on the earth 
complete from end to end. But our phylogenetic hypo- 
theses are entitled to ask for recognition equal to that 
accorded to these generally accepted geological hypo- 
theses. The only difference is, that the mighty theories 
of geology are far more complete, simpler, easier of 
comprehension than is that of the younger science, phy- 
logeny. 13 

Thus, these historical natural sciences, geology and 
phylogeny, form the connecting band between the 
exact natural sciences on the one hand, and the histori- 
cal metaphysical sciences on the other. Biology, as a 
whole, but especially systematic zoology and botany, 
is therefore raised to the rank of a true natural history, 
an honorable title, long sought by these departments 
of knowledge, but only of late deserved. The fact that 
these same branches of knowledge are still, at the 
present time, described in many places and even 
officially, 14 as " descriptive natural sciences," and are 
placed in antagonism to the " explanatory," only serves 
to show the false conception hitherto held as to their 
province. Since the natural system of organisms has 
been recognised as their genealogical tree, in the place 
of the dead, descriptive systematising, stands erect and 
living the ancestral history of classes and species. 

Highly as we value this immense advance of mor- 
phology, it would still, not of itself, be sufficient to 
explain the extraordinary effect of the modern theory 
of Evolution upon knowledge as a whole. This 


depends rather, as will be admitted, upon one single 
special conclusion involved in the theory of descent, 
upon its application to man. The time-honored ques- 
tion as to the origin of our own race, is by this theory 
answered for the first time in a scientific manner. If 
the theory of Evolution is true as a general proposition, 
if it, as a whole, gives a natural history of the ancestry 
of living things, then is man the crown and summit of 
the creation, "the paragon of animals," sprung from the 
phylum of the Vertebrata, the class Mammalia, the sub- 
class, Placentalia, the order Quadrumana. Since Linnaeus, 
as early as 1735, in his fundamental system of nature, 
united man with the apes and bats in the order Pri- 
mates, as all zoologists since then have been unable 
to separate him from the class of mammals, it follows 
that this systematic grouping of man that is now fol- 
lowed with one voice by all zoologists, can only find 
its phylogenetic explanation in the origin of man from 
that class. 15 

All attempts to do away with this consequence of 
the theory of Evolution, with all its fulness of mean- 
ing, are in vain. It is in vain that men try to preserve 
a special place for man, in vain that they construct for 
him a line of ancestors of his own alone, separately 
from the genealogical tree of the Vertebrata. The 
phylogenetic proofs of comparative anatomy, ontogeny, 
palaeontology, speak out too clearly in favor of a uni- 
form origin for all Vertebrata from one single common 
stem-form for us to be able any longer to be in doubt 
at the present day. Not one comparative philologist 
holds it as probable that languages so different as the 
German, Russian, Latin, Greek, Hindustani, have 
evolved from different primitive languages. On the 
contrary, all linguists, after critical comparison of the 
structure and development of these different languages, 
agree in the conclusion that they have all originated 
from one single Aryan or Indo-European primal 
speech. 16 In exactly the same way, to all morpholo- 
gists the firm conviction has come that all the Verte- 
brata, from Amphioxus up to man, all Pisces, Amphi- 
bia, Reptilia, Aves, Mammalia, arose originally from 
a single vertebrate form. For it is unthinkable that 


all the different and highly complex conditions of 
existence that led through a long series of processes 
of Evolution to the formation of the vertebrate type, 
could have concurred more than once in the course of 
earth's history. 

As for us to-day only the general conception of the 
vertebrate origin of man is of moment, we will not 
pause longer over the individual details of our ancestral 
tree. 17 We need only, in regard thereto, point out in 
passing that the chief stages in the series at least are 
already at the present time assured, thanks to the 
splendid labors of our most renowned morphologists, 
and in especial to those of Gegenbauer and Huxley. 18 
Still it is often asserted to-day that by these labors 
only the origin of the human body, but not that of our 
minds, is explained. In answer to this important 
objection, we must keep ever in mind the physiological 
fact that our soul-life is indissolubly bound up with 
the organisation of our central nervous system. But 
the latter is composed and is developed in exactly the 
same manner as is that of all the higher Vertebrata. 
Further, according to the researches of Huxley, the 
differences between man and the higher apes in brain- 
structure are far less than the corresponding differences 
between the lower and the higher apes. As the func- 
tion or work of every organ is unthinkable apart from 
that organ, as function is everywhere developed hand 
in hand with the organ, we are forced to the conclusion 
that our mental functions have slowly and with steady 
ascent evolved pari passu with the phylogenetic up- 
building of our brain. 

Further, this question of soul, so full of significance, 
appears to us at the present moment in a light alto- 
gether other than was its wont twenty or even ten 
years ago. As man has gained a conception of the con- 
nexion between soul and body, spirit and matter, so 
the modern theory of Evolution goes with its clearer 
eyesight much further, and teaches us that all organic 
matter at least, if not matter as a whole, is, in a sense, 
gifted with a soul. In the first place, improved micro- 
scopic investigation has taught us that the structural 
elements of organisms, the cells, have, as a rule, an 


individual soul-life. Since Schleiden, forty years ago, 
established the cell-theory as far as the plant kingdom 
is concerned, and Schwann, in like manner, extended 
it to the animal kingdom, we ascribe to these micro- 
scopic living beings an individual, independent life. 
They are the true " individuals of the first rank," or 
" elemental organisms" of Brucke. The magnificent 
and fruitful application of the cell-theory made by 
Virchow, in his cellular pathology, to the domain of 
theoretical medicine rests on the fact that the cells are 
no longer to be regarded as dead, passive, building- 
stones in the organism, but as its living, workful 

This conception is ultimately based upon the study 
of the Infusoria, AmoebaB, and other unicellular 
organisms. For here, in the solitary, isolated cells, 
we meet with the same outcome of soul-life, sensa- 
tion and consciousness, volition, motion, as in the 
higher animals composed of many cells. But in these 
latter cell-societies, as in those former solitaries, the 
soul-life is linked on to, is bound up in, one and the 
same cell-substance of first importance, protoplasm. 
In the Monera and other very simple organisms, we 
see that single, detached particles of protoplasm have 
sensation and movement, as have complete cells. 
Further, we are compelled to conclude that the cell- 
soul, 19 the fundamental of empirical psychology, is in 
its turn complex, is, in short, the resultant of the 
psychic properties of the protoplasmic molecules that 
we name, in brief, plastidules. The soul of the plas- 
tidule 20 would, therefore, be the ultimate factor of the 
organic soul-life. 

Has our modern theory of Evolution exhausted with 
this its psychological analysis ? By no means. The 
more recent organic chemistry, on the contrary, teaches 
us that it is the peculiar physical and chemical pro- 
perties of one element — carbon — that, in complex union 
with other elements, condition the peculiar physiolo- 
gical properties of organic compounds, and especially 
of protoplasm. The Monera, consisting simply of pro- 
toplasm, bridge over the deep chasm between organic 
and inorganic nature. They show us how originally 


the simplest and most ancient organisms must have 
sprung from inorganic carbon compounds. If origin- 
ally a definite number of carbon atoms united with a 
number of atoms of hydrogen, oxygen, nitrogen, sul- 
phur, into a plastidule or protoplasmic molecule, we 
must regard the plastidular soul, i.e., the sum of its 
life-functions, as the necessary product of the forces of 
those conjoined atoms. But the sum of the central 
atomic forces we may call, in the strict Monistic sense, 
an " atom-soul." 21 From the accidental collision and 
manifold combinations of the constant, unchanging 
atom-souls arise the manifold and highly variable plas- 
tidular souls, the molecular factors of organic life. 

Related to this extremest psychological consequence 
of our Monistic theory of Evolution, is that old concep- 
tion of the possession of a soul by all matter that finds 
varying expression in the philosophies of Democritus, 
Spinoza, Bruno, Leibnitz, Schopenhauer. For all soul- 
life admits, ultimately, of being reduced to the two 
elementary functions, sensation and movement, and to 
their reciprocal action in reflex movements. The 
simple sensations of pleasure and pain, the simple 
movements of attraction and repulsion, are the real 
elements out of which all soul-function is built up in 
its endless variety and complexity. The " loves and 
hates " of atoms, the attraction and repulsion of mole- 
cules, the movement and sensation of cells and of the 
organisms made up of cells, the thought and conscious- 
ness of man 22 — all are but different steps in the uni- 
versal process of psychological evolution. 

The unity of phenomena (or Monism) to which, 
therefore, the new theory of Evolution leads us, does 
away with the opposition hitherto existing between 
the two different dualistic systems of the universe. 
It avoids the one-sided policy of Materialism and of 
Spiritualism alike : it connects practical idealism with 
theoretical realism; it binds together physical and 
metaphysical science in one all-embracing, unified 
knowledge of things. 

Whilst at the present time we see in the theory of 
Evolution a connecting bond between branches of 
knowledge the most diverse, this same theory is of 


first importance, not only for the purely theoretical, 
but also for the practical, applied disciplines. Neither 
practical medicine, an applied natural science, nor 
practical statecraft, jurisprudence, and theology, in so 
far as they are portions of applied philosophy, will be 
able to withdraw themselves from its influence. We 
are rather of conviction that in all these departments it 
will, as a rule, prove itself to be the most important 
instrument, both for advancing knowledge and for the 
better culture of man as a whole. For, as the most 
important means towards culture is the education of 
the young, the theory of Evolution must have the influ- 
ence that is its due even in the school. Herein it is 
not to be on sufferance merely. It must receive its due 
proportion of attention — it will become our guide. 

If, in conclusion, we are allowed in the fewest words 
possible to indicate the weightiest point in this relation- 
ship of Evolution to general knowledge, the great sig- 
nificance of the genetic method ought really to be dwelt 
on first. Teacher and learner, alike, will regard every 
subject of study with ever-growing interest and under- 
standing, if they ask themselves these questions before 
all others. How has it arisen ? How has it evolved ? 
For in this question as to Evolution, the question as to 
the causes of facts is involved ; and it is always the 
knowledge of efficient causes, not the mere knowledge 
of the facts, that satisfies the constant desire in respect 
to causality that our reason has. The knowledge of 
general, simple primary causes for the most different, 
the most complex phaenomena leads at once to simpli- 
fication and to intensification of our knowledge. It is 
only by understanding of causes that our dead know- 
ledge becomes living science. The true measure of 
our mental culture is not the quantity of our empirical 
knowledge, but the quality of our understanding as to 

How far the fundamentals of the general theory of 
Evolution have already made their way in our schools 
— in what order its chief branches, cosmogony, geo- 
logy, the phylogeny of plants and of animals, anthro- 
pogeny, should be learned in the various classes — the 
determination of these points we must leave to the 


practical schoolmaster. But we believe that a great 
reform of study in this direction is inevitable, and that 
it will be followed by the happiest results. For 
example, how immeasurably will the important study 
of language gain in value when it is treated compara- 
tively and genetically! How the interest in physical 
geography will grow when that branch of knowledge 
is linked on genetically with geology ! What light and 
life will the systematic arrangement of organic species 
gain when once these are explained as the different 
branches of one common ancestral tree ! And, above 
all, what a new understanding will be ours of our own 
organism when we no longer see it in the dim, magic 
mirror of mythology as the pretended image of an 
anthropomorphic creator, but behold it, in the clear 
daylight of phylogeny, as the most highly-developed 
form of the animal kingdom ; as an organism that has 
gradually evolved in the course of many millions of 
years from the ancestral series of Vertebrata, and has 
far outstripped all its fellows in the struggle for exist- 

When, therefore, the theory of Evolution, fertile, 
helpful, enters into all branches of study, it will 
awaken in teachers and scholars the consciousness of 
its unifying power. As a historical, natural science, it 
will pass as mediator, as reconciler, between the two 
opposed orders of thought that are to-day contending 
for mastery in the higher school-culture : on the one 
hand, the older classical, historical and philosophical 
learning ; on the other, the newer, exact, mathematical 
and physical studies. We regard both kinds of study 
as alike necessary, alike indispensable. The mind of 
man will only reach its full, harmonious completion, 
when both are dealt with. But just as in earlier days 
classical lore was exclusively studied, so is it to-day 
only too frequently, with exact learning. The theory 
of Evolution leads both these excessive ideas back to 
the happy medium, whilst it acts as the connecting 
bond between exact and classical, physical and meta- 
physical science. It demonstrates everywhere the 
living stream of connected, unified, unbroken Evolu- 
tion. Ever is it showing to the eager student new 



goals of science beyond those already gained, drawing 
" gently the striving heart nearer to the truth." The 
endless perspective of advancing improvement thus 
opened out before us by the theory of Evolution is 
withal the best protest against the fatal " ignorabimus," 
that we hear sounding on all sides of us to-day. 23 For 
none can say what " limits of natural knowledge " the 
mind of man shall transcend hereafter, in the widening 
scope of its astounding development. 

By far the most important and weighty claim that 
practical philosophy makes upon the theory of Evolu- 
tion would seem to be that it act as a new theory of 
conduct. Beyond a doubt, hereafter, as aforetime, the 
careful building up of the moral character, and of 
religious conviction, must remain as the highest aims 
of all education. Until the present hour the vast 
majority of men were firm in their conviction, that this 
most momentous end was possible only in relation to 
certain creeds of certain churches. But as these dogmas, 
especially in regard to the old creation myths, are 
directly contradicted by the facts of Evolution, it is the 
belief of many that by this last religion and morality 
are endangered. 

We hold this fear as false. It arises from the peren- 
nial confusion between true, reasoning, natural religion, 
and the dogmatic, mythological religion of the churches. 
The comparative history of religion, an important 
branch of anthropology, teaches us to recognise the 
great variety of the external forms in which different 
peoples and different ages clothe, according to their 
individual characters and needs, their religious thought. 
It shows us that even the dogmatic teachings of the 
religions of the churches are linked together in one 
gradual, unbroken stream of Evolution. New churches 
and sects arise. Old ones pass away. Under the most 
favorable conditions, any particular order of thought 
holds its own but a poor two thousand years, a short, 
evanescent time in the succession of aeons of geological 
time. Finally, the history of civilisation teaches us 
how little real morality is, in essence, connected with 
any particular form of ecclesiastical creed. Often hand 
in hand with the absolute lordship of an almighty 



church, go the grossest barbarity and lawlessness. Let 
us think only on the mediaeval age ! And on the other 
hand, we see men, who have cast themselves away from 
every belief of the churches attaining the loftiest 
heights of moral grandeur. 

Dissevered from all church creeds, there lives in the 
breast of every man the germ of a true natural religion. 
It is indissolubly bound up with the noblest parts of 
man's being. Its highest commandment is love, the 
narrowing down of our natural egoism in favor of 
our fellows, for the good of that human brotherhood 
of which we are members. This natural law of morals 
is far older than all the religions of the churches. It 
has evolved out of the social instincts of animals. 2 * 
Amongst animals lower than ourselves, we encounter 
the beginning of these instincts in very different classes, 
especially in mammals, birds and insects. Following 
the laws of association and of the division of labor, many 
persons, in these cases, band themselves together into 
the higher community of a state. The nature of that 
state is, of necessity, dependent upon the mutual inter- 
working of its members, and on the sacrifices of egoism 
that each makes to the community as a whole. Con- 
sciousness of this necessity, the sense of duty, is none 
other than a social instinct. But instinct is in all cases 
a psychical habit, that, originally acquired by adaptation, 
has become hereditary in the course of generations, and 
at last appears to be " innate." 

To convince ourselves as to the marvellous power of 
the sense of duty in the lower animals we have but to 
knock to pieces an ant-hill. Then we see at once in the 
midst of the catastrophe thousands of busy citizens not 
concerned with the salvation of their own dear lives, 
but with the protection of the community to which 
they belong. Brave warriors of the ant-state oppose 
themselves in forceful antagonism to our intrusive 
finger. The nurses of the young within rescue the 
so-called " ant-eggs," the beloved pupae, upon whom 
the future of the state depends. Busy workers begin 
at once with indomitable courage the rebuilding of 
the wrecked home, the construction of new dwelling- 
places. But the marvellous civilisation of these ants, 



of the bees, and of other social animals, has originally 
evolved from the crudest of beginnings, even as has 
our own human civilisation. 

Even those relationships of human life that our poets 
extol most highly are found foreshadowed even among 
the kingdoms of the lower animals. Have not the 
deep maternal love of the lioness, the touching love of 
the mate in the Java sparrows (the inseparables), the 
devotion and faith of the dog, become this long time 
proverbial ? In these, as in man, the noblest affections 
of sympathy and of love that determine conduct are no 
other than bettered instincts. In consonance with this 
idea the ethics of Evolution have to seek out no new 
fundamentals. They have rather to lead the ancient, 
primary sense of duty to its natural and scientific basis. 
Long ere the origin of all church religions this natural 
sense of duty, of responsibility, ruled the law-abiding 
life of men, as of the lower social animals. And this 
immense fact the ecclesiastical religions will do well 
to utilise, in place of contending against it. For the 
future belongs not to that theology which wages a 
hopeless war against the victorious truth of Evolution, 
but to that which grasps, understands, and makes use of 
that truth. 

Far from fearing as result of the influence of Evolu- 
tion on our religious convictions, a destruction of all 
moral laws, and the terrible license of unbridled 
egoism, we hope rather, as consequence of Evolution, 
for a rational foundation of morals on the unim- 
pregnable basis of fixed natural laws. For with the 
true understanding of our true place in nature, An- 
thropogeny opens up to us at the same time a glance 
into the necessities of our primary social responsibili- 
ties. Practical philosophy and pedagogy, like general 
theoretical knowledge, will henceforward derive its 
most important fundamentals, not from pretended reve- 
lations, but from the natural truths of Evolution. This 
conquest of dualism at the hands of Monism reveals to 
us a future full of hope, in which shall be an endless 
advance of our moral and of our intellectual develop- 
ment. In this sense we hail the modern truth of Evo- 
lution founded in these later days by Charles Darwin, 


as that which more than all others has carried forward 
all our knowledge, pure and applied. 


(1) Page 278. — Charles Darwin in his chief work (" The 
Origin of Species through Natural Selection," 1859) has already 
stated and explained all the chief points in his own theory of 
Evolution, save in its application to man. This latter was 
first dealt with in 1871 in the work on 44 The Descent of Man 
and Sexual Selection." His other writings only contain further 
extensions and more definite and detailed proofs of the funda- 
mental propositions laid down in those two main works. 

(2) Page 278. — On the relation of Immanuel Kant to the 
theory of Evolution, see Fritz Schulze : 44 Kant and Darwin, a 
Contribution to the History of the Evolution Theory" (1875). 
On the bearing of the writings of Wolfgang Goethe upon 
Evolution generally, see my 44 Naturliche Schopf ungs-ges- 
chichte " (6th edition, p. 73). 

(3) Page 278. — The zoological philosophy of Lamarck (pub- 
lished in 1809, and very soon translated into German by 
Arnold Lang) is the only work before Darwin's time (he was 
born in 1809) that attempted to deal with the domain of bio- 
logical development in relation to and on the principles of 
mechanical phenomena ; a very grand if premature attempt. 

(4) Page 278. — Lorenz Oken's services to Evolution are 
generally so inaccurately gauged that most stress is laid on 
the phantastic excrescences of his writings on natural philo- 
sophy. On the contrary we ought to remember that he not 
only laid down the fundamental principle of the unity of 
development of the universe, but even anticipated the prin- 
ciples of the cell and protoplasm theories, and that he was the 
first in our century to undertake observations in embryology 
(Kesearches on the formation of the alimentary canal, 1806). 
See 44 Naturl. Schopf.," 6th edition, p. 86. 

(5) Page 279. — It is amongst the most remarkable phseno- 
mena in the history of science that the supernatural theory of 
catastrophes laid down by Cuvier actually held its own until 
the last thirty years in biology, when that science was advanc- 
ing so rapidly. And this, despite the fact that the anta- 
gonistic theory of uniformity had been stated by Lamarck as 
early as 1809, and had, since 1830, been shown by Lyell to be 
fully confirmed by geology. 44 Naturl. Schopf.," 6th edition, 
pp. Ill, 115. 


(6) Page 279. — When, fourteen years ago, at the thirty- 
eighth meeting of scientific men at Stetting, Sept. 12, 1863, I 
gave a lecture on the "Darwinian Theory" (see p. 3), and in 
doing thus gave open expression to his teaching for the first 
time in such an assembly, the recognition of that teaching 
was decisively refused by the great majority of my hearers. 
To-day such recognition is given willingly enough by all com- 
petent naturalists. See my preface to the fourth edition of 
" Naturl. Schopf." 

(7) Page 280. — Chorology, the study of the geographical 
and topographical distribution of organisms, and oecology, the 
study of the household, of the habits of life of organisms and 
their relations one to another, are physiological disciplines. 
They do not prove so directly as morphological studies the truth 
of the theory of descent. But their general phaenomena can 
only be explained by the aid of that theory. See lecture xiv., 
44 Naturl. Schopf." 

(8) Page 280. — Dysteleology, or the study of the undesigned, 
is the name given to the study of the rudimentary organs, in 
that these organs in simpler and in clearer manner than all 
other phaenomena, contradict that widely-spread teleology, or 
the study of design, that holds such sway in dualistic philo- 
sophy. " Gren. Morph.," vol. ii., p. 266. 

(9) Page 280. — Crystallography and the promorphology of 
organisms may, e.g., be called exact morphology. For both 
seek to refer the actual forms of bodies (in the one case the 
crystals, in the other the organic individuals), to ideal geo- 
metrical fundamental forms. But by far the greater part of 
morphology, and also a large part of physiology (e.g. chorology, 
oecology, psychology) are in large measure incapable of 
mathematical treatment, and are, therefore, not exact. 

(10) Page 282. — The fundamental biogenetic law runs more 
accurately as follows : The development of the embryo (on- 
togeny) is a condensed and abbreviated repetition of the evolu- 
tion of the race (phylogeny). This repetition is the more 
complete, the more the true original order of evolution (palin- 
genesis) has been retained by continual heredity. On the 
other hand this repetition is the less complete, the more by 
varying adaptations the later spurious development (ceno- 
genesis) has obtained. " Anthrop." 3rd. edition, p. 11. 

(11) Page 282. — The cenogenetic disturbances that are 
brought about in the succession of palingenetic development 
by adaptation of embryos to the conditions of embryonal life, 
are in great part alterations of relations of place and of time 
in development (heterotopia, heterochronia), are in part new 
embryonic structures (e.g., the formation of the egg shell, the 
amnion, etc.). 


(12) Page 282. — The historical character of the morphologi- 
cal sciences (especially of comparative anatomy and ontogeny, 
as of palaeontology), cannot be dwelt upon too much. To 
aim at the exact description of empirical facts, is here as 
natural as it is in history itself. But these sciences themselves 
can never be exact. 

(13) Page 283. — Geology and phylogeny not only pursue 
kindred aims. They use the same methods. In both disci- 
plines, the end is by thoughtful comparison of many separate 
facts, by critical examination into their historical meaning, by 
speculative filling up of the gaps left after all observation, to 
build up a connected account of the history of Evolution in the 
one case of the earth, in the other of its inhabitants. " Anthrop." 
3rd edition, pp. 329, 382. 

(14) Page 283. — The biological disciplines are to-day called 
officially (e.g., in the Prussian examination regulations), de- 
scriptive sciences as opposed to physics and chemistry. This 
designation contains a contradictio in abjecto. For a true science 
can never be merely descriptive. Besides, in botany as in 
physics and chemistry, in morphology as in physiology, the 
empirical description of facts is only the beginning, their 
causal explanation is the philosophic end of science. 

(15) Page 284. — The origin of man from other mammals, 
and most directly from the catarrhine apes, is a deductive law, 
that follows necessarily from the inductive law of the theory 
of descent. " Anthrop." 3rd edition, p. 392. 

(16) Page 284. — August Schleicher on " The Darwinian 
Theory and the Science of Language," 1863. 

(17) Page 285. — The ancestral series of man, which the xvi. 
— xix. lectures of my " Anthropogenic " sketch, is not less or 
more scientifically certain than all other phylogenetic and 
geological hypotheses, even if the different stages are not of 
equal certainty. When Du Bois-Reymond (Darwin versus 
Galiani), 1876, sneers that " ancestral trees of our race, 
sketched in the ' Schopfungs-geschichte,' are of about as 
much value as are the pedigrees of the Homeric heroes in the 
eyes of the historical critic," he but shows his astonishing 
ignorance of the morphological investigations on which these 
trees are based. If he calls phylogeny " a romance " he must 
give the same name to geology. 

(18) Page 285. — For a knowledge of the vertebrate ancestors 
of man, the " Researches in the comparative anatomy of the 
Yertebrata " of Carl G-egenbauer, a work as thorough as it is 
critical, is of great value. See also his " Elements of Com- 
parative anatomy." 

(19) Page 286. — The cell-soul in the Monistic sense, is the 
totality of the responsive forces resident in the protoplasm. 


The cell-soul is as indissolubly bound up with the protoplasmic 
body as is the human soul with the brain and spiral cord. 

(20) Page 286. Plastidule-souls. The plastidules or proto- 
plasmic molecules, the smallest, homogeneous parts of the 
protoplasm are, on our plastid theory, to be regarded as 
the active factors of all life-functions. The plastidular 
soul differs from the inorganic molecular soul in that it pos- 
sesses memory. See my plastid-theory in the 44 Studies on 
Monera and other Protista," (1872), and also the previous 
lecture on the Perigenesis of the plastidule, or the wave-motion 
of living particles. 

(21) Page 287. — Atom-souls. The recent contest as to 
the nature of atoms, which we must regard as in some 
form or other the ultimate factors in all physical and che- 
mical processes, seems to be capable of easiest settlement 
by the conception that these very minute masses possess, as 
centres of force, a persistent soul, that every atom has sensation 
and the power of movement. See also G-ustav Tschermark's 
44 Unity of Development in Nature," Yienna, 1876. Zollner on 
the 44 Nature of Comets," Leipsic, 1872. 

(22) Page 287. — Consciousness, since the lecture of E. Du 
Bois-Reymond, in 1872, to the 45th meeting of German 
naturalists at Leipsic, has been very generally held as an im- 
passable limit of natural knowledge and as, therefore, a second 
boundary altogether different in its nature from the first, or 
the connexion between matter and force. But beyond a doubt 
these two ideas are one and the same, although Du Bois-Rey- 
mond sneers that 44 we cannot, on this point, come to any 
decision, and must abandon all further inquiry thereon " (1. c. 
p. 33). Little as we are in a position, at the present time, to 
explain fully the nature of consciousness, yet the comparative 
and genetic observation of it clearly shows, that it is only a 
higher and more complex function of the nerve-cells. 

(23) Page 290.— The " ignorabimus," which E. Du Bois- 
Reymond, in the lecture just cited (note 22), places in the 
way of the advance of our knowledge, is now on every oppor- 
tunity quoted by the foes of Evolution as testimonium paujper- 
tatis of natural science. We, therefore, in this place, as already 
in the preface to these lectures, directly protest against it. 
For the study of the evolution of soul-life shows us that this 
has worked its way up from the lower stages of the simple 
cell-soul, through an astonishing series of gradual stages in 
evolution, up to the soul of man. No one, therefore, has the 
right to hold that in the future we shall not be able to pass 
beyond these limits of our knowledge that to-day seem impas- 
sable. Darwin, in the introduction to his 44 Descent of Man," 
says : 44 It is always those who know little, and not those who 



know much, that positively affirm that this or that problem 
will never be solved by science." 

(24) Page 291. — The social instincts of the lower animals 
have, of late, been regarded for various reasons as clearly the 
origin of morals, even of those of man. The laws of associa- 
tion and division of labor, in the one case as in the other, bring 
about the mutual inter-working of individuals in combination. 
This leads to the sense of duty, to responsibility. Therefore 
the history of civilisation in animals, a field of zoology as yet 
almost un worked, will have as its aim the tracing back of the 
civilisations of the ants, the bees, and other animals that live 
in communities to the lower, cruder historical conditions, just 
as in the history of the civilisation of man. 




THE knowledge of historical development is at the 
present time rightly regarded as the surest way 
to the true understanding of organic bodies. Espe- 
cially is that knowledge of importance in regard to 
those organs, that through all their complex structure 
seem to be fashioned upon a determinate common 
plan. Nowhere does such a systematic, definite ar- 
rangement meet us so plainly as in our sense-organs. 
The exquisite structure of our eye, the marvellous 
labyrinth of our ear, have no equals in other organised 
bodies. Hence they have always been the especial 
favorites of anatomical and physiological investigation. 
Moreover this preference is intensified by the incom- 
parable significance of these, the most important instru- 
ments of the mind. For the sense-organs are the sole 
sources of all knowledge, the only doors through which 
the external world makes entry into our inner mental 
life. Hence speculative philosophy has ever taken a 
special interest in this part of biology, for in this realm 
it comes into the most active relationship with empi- 
rical investigation. 

Whilst at the present time the theory of Evolution, 
based on the firm foundations laid by Darwin, makes 
claim to explain the origin and formation of the sense- 
organs in the same way as it explains those of other 
organs, by the slow, gradual process of Evolution 
resulting from Natural Selection, it must be admitted 
that it is encompassed at the outset with the greatest 
difficulties. To conquer these nothing is of greater 
value than a brief glance at the history of the indi- 
vidual germ. For if we see that in any individual 



animal body these organs do not at first exist, but are 
developed slowly and gradually, then this important 
fact in the history of the germ is of use in giving us the 
key to the solution of the far more difficult, far more 
obscure question as to the evolution of these organs in 
the past. 

To convince ourselves of this momentous truth, we 
need but place a hen's egg in a hatching machine, and 
follow out step by step in the brief space of three 
weeks the building up of a simple germ into the per- 
fect bird. Thus we can settle by direct observation 
that eye, ear, even the lower sense-organs of smell 
and taste, are not present at the commencement of the 
development of the germ. They make their first 
appearance later on, and from a commencement undif- 
ferentiated, and of the greatest simplicity, pass gradu- 
ally through a series of very wonderful changes to 
their later structure and form. Fifty years ago this 
fundamental fact was established by C. E. von Baer, the 
great embryologist, who raised the account of the in- 
cubated hen's egg to the position of one of our most 
important sources of knowledge. Next the able biolo- 
gist, Emil Huschke, of Jena, a few years later (1830) 
worked out more fully, with extensive care, the won- 
drous details of these great changes. Many observers, 
startled by his brilliant discoveries, have more recently 
worked these out with a completeness that is astonish- 
ing. Finally, the general conclusion has been reached, 
that in man and in all other animals the sense-organs 
as a whole arise in essentially the same way, viz., as 
parts of the external integument or epidermis. The 
external integument is the original, general sense- 
organ. Gradually the higher sense-organs detach them- 
selves from this their primal condition, whilst they 
withdraw more or less completely into the protecting 
interior of the body. Nevertheless in many animals, 
even at the present hour, they lie in the integument, as 
e.g. in the Vermes (Fig. 50). 

But the activity of the sense-organs, as of all others, 
depends entirely upon that of the microscopic cells 
composing them. These small cells are, in fact, the 
true, independent " elemental organisms," whose func- 


tions in their continued totality condition the life of 
the whole multicellular organism. Hence the sense- 
organs, the sense-cells that give rise to the various 
sensory consciousnesses, are of greatest moment : the 
visual cells of the eye, the auditory of the ear, the 
olfactory of the nose, the gustatory of the tongue. If, 
then, as we now know, all the various sense-organs are 
actually only particular parts of the integument spe- 

Fig. 50. 

Nervous system and sense organs of the Turbellaria. Two 
kinds of nerves radiate from the simple nerve -ganglion or 
brain (g) : the centripetal sensory nerves (s) pass to the 
skin (h), tentacles (£), auditory vesicles (o), eyes (a); the 
centrifugal motor-nerves (m) go to the flesh, the sub-mus- 
cular epidermal layer (/) ; w cilia of the epidermis. 

cialised and modified, all the various sense-cells must 
have originally arisen from single epidermal cells. 
They are, in fact, as a whole transformed descendants, 



specialised in different ways, of the epidermal cells that 
are for the most part undifferentiated (Fig. 51). 

This fundamental fact, whose significance cannot be 
valued too highly, is now established beyond a doubt. 
Every one who studies the incubated egg by the aid of 

Fig. 51. 

Epidermal cells of a human embryo of two months. 

a good microscope, and the perfected methods of inves- 
tigation now known, can convince himself that all sense- 
organs take origin from the integument. If we study, 
e.g., the embryo of a chick on the third or fourth day 
of incubation (Figs. 52 — 56) we find that the earliest 
appearance of the nose (n), the eye (Z, sp), the ear (o), 
is a simple depression of the integument. But this 

Fig. 52. Fig. 53. 

Head of a chick-embryo (third day of incubation). 1. An- 
terior view. 2. From the right hand side. n. Rudimentary 
nose (olfactory depression). I. Rudimentary eye (ocular 
depression), g. Rudimentary ear (auditory depression). 
v. Fore-brain, gl. Eye-cleft, o. Superior maxillary pro- 
cess, u. Inferior maxillary process of the first gill-arch. 



holds in regard to all the sense-organs of all other 
animals, and even of man. By this pregnant fact not 
only is the question as to the origin of the sense-organs 
greatly simplified. The key to the true understanding 
of them is also given. For by a fundamental biolo- 

Fig. 54. 

Head of a chick-embryo (fourth day), from below, n. Nasal 
groove, o. Superior maxillary process of the first gill-arch. 
u. Inferior maxillary process of the same. k. Second gill- 
arch, sp. Choroidal clef fc of the eye. s. (Esophagus. 


Fig. 55. Fig. 56. 

Two heads of embryonic chicks. 1, at the end of the fourth ; 
2, at the beginning of the fifth day. Letters as in Fig. 54, 
and in addition in internal, an external nasal process, nf. 
Nasal furrow. sL Frontal process, m. Oral aperture. 

gical law, the general law of organic evolution, every 
embryological fact is in direct, causal relation to a 
corresponding genealogical event that took place long 
previously, thousands, perhaps millions, of years ago, 




in the history of the ancestral series of the particular 

Originally such a change occurred in the ancestral 
history by adaptation of earlier forms. Then by here- 
dity it was transmitted from these with greater or less 
completeness down the long line of descendants. If 
we see in the early stages of the chick in the incubated 
egg that the higher sense-organs are at first wanting, 
and that the first trace of them appears in the epidermis, 
we conclude that the earlier ancestors of the bird were 
lower animals, destitute of either eyes or ears, and that 
later on special regions of the epidermis in their 
descendants learned for the first time to distinguish 
light-waves and sound-waves. And if further we see 
that the delicate organs for the perception of the finer 
colors and sounds, the rods in the retina of the eye, the 
fibres of Corti in the cochlea of the ear, appear much 
later in the embryo of the bird, after the other parts of 
the eye and ear have been formed, we conclude that 
these most delicate and most perfect sense-instruments 
were acquired at a much later period in the earth's 
history by some more recent ancestor of the birds. 

It must be remembered that this important con- 
clusion as to the relationship between the observed 
history of the individual embryo and the hypothetical 
history of its forefathers is not applicable universally 
and without limitation. Everything that happens in 
the embryo does not allow of an explanation in relation 
to ancestral evolution. But where this is not admis- 
sible, when important gaps and breaks interrupt the 
chain of historical development, another science comes 
to our aid, that of comparative anatomy. This fascinat- 
ing science compares the structure of the completed 
organs in the various classes and orders of animals. It 
proves that those organs are to be found side by side in 
the different animal groups in most varying stages of 
development. It gives a very instructive glance at the 
long series of historical evolution by which these organs 
have gradually, one after another, worked upwards 
from the simplest commencement to the highest com- 
plexity. Thus comparative anatomy, in quite another 
fashion than embryology, shows us that the wondrously 



complex structure of our human eye and ear is con- 
nected by a long, long series of intermediate forms, 
with the simpler and with the simplest organs of sight 
and hearing in the lower animals. Whilst the general 
arrangement of these organs is essentially the same in 
the higher Vertebrata, the Mammalia, Aves, Reptilia, 
as in man, we encounter simpler conditions in the 
Amphibia, and yet simpler in Pisces. But if we com- 
pare with these last the corresponding sensory structures 
in the lower animals, especially in Vermes, we are 
convinced that even the imperfect eyes and ears of 
Pisces are the later product of a long series of improve- 
ments and advancing perfection, through which these 
physical instruments have passed during many millions 
of years in the invertebrate ancestors of the fishes. 

If, now, aided by these very important evidences as 
to descent, on the one hand by comparative anatomy, 
on the other by embryology, we try to make out the 
historical evolution of the sense-organs in man and in 
other animals we must first call to mind certain diffi- 
culties and certain precautionary measures, that must 
ever be kept in view in these difficult historic questions. 
For example, we can only decide as to the sensory im- 
pressions of other beings by the impressions that we 
ourselves receive through our own sense-organs. Hence 
we can have no conception of sense-functions that we 
cannot ourselves exercise. As the man born blind can 
have no idea of the nature of colors, as the man born 
deaf and dumb can have none of the nature of sounds, 
man can have no idea at all of these sense-functions in 
other animals that are wanting in man himself. 

Five different sense-organs are generally distin- 
guished in man. Of these the lowest is the integument, 
performing two different sensory functions, as the 
organ of touch and the organ for the perception of 
temperature. The tongue and the nose, as organs of 
taste and of smell, rise to an intermediate position, 
whilst ear and eye, the aesthetic organs of hearing and 
of sight, reach the highest condition of perfection. But 
comparative anatomy and physiology teach us, that 
with these six different kinds of human sense-functions 
the range of the sensations in the animal kingdom is 




by no means exhausted. On the other hand, we know 
in various animal classes organs of complex structure, 
with peculiar terminal organs of a nervous nature, that 
seem undoubtedly sense-organs, but do not seem to 
belong to any of the senses known to us. Such organs 
of a sixth or seventh sense, not known to us, are e.g., 
the cup-shaped nerve-organs in the skin of many 
Vermes, the gelatinous tubes and mucous canals with 
peculiar nervous enlargements and cups in the skin of 
fishes (Fig. 57). It may be that such organs make pos- 

Fig. 57. 

Two cup-shaped sense-organs (b) of unknown value in the 
skin of the tench (Tinea), n. Sensory nerves in the cuticle 
running into the longitudinally striated sensory cells of the 
cup (b). Between the latter are ordinary rounded epidermal 

sible to these water-dwellers the perception of certain 
conditions of the water of which we know nothing. 

In other cases we ascribe the strange actions of cer- 
tain animals, that do not appear due to senses known 
to us, to the presence of sense-organs of which we are 
ignorant. Were we to repeat Spallanzani's cruel experi- 



ment, and let bats whose eyes and noses were destroyed, 
whose ears were stopped with wadding, fly about a 
room across which many strings were stretched, the 
mutilated animals, despite their injuries, would fly dex- 
trously in and out among the cords without touching 
them. In this case either there must be a special un- 
known sense-organ at work, or the sense of touch or of 
temperature is so exalted quantitatively, that it would 
seem to be a specialised and peculiar sense qualitatively. 
Further, the sense of locality in migratory birds and 
carrier-pigeons, as well as many so-called " enigmatical 
instincts " in the lower animals are most easily explic- 
able on the supposition of a special sense-organ. There 
are, in all probability, many unknown qualities of 
natural bodies, of which we have absolutely no idea, 
inasmuch as the organs for perceiving them are want- 
ing in us. The limits of our knowledge are bounded 
by those of our sensory perceptions. 

In inquiries such as these, w r e must always bear in 
mind the fundamental fact that we do not perceive by 
our sensations the veritable properties of natural ob- 
jects. All of which we are cognisant is only the 
occasional conditions of our sense-organs stimulated 
in special way by pressure, temperature, sound-waves, 
light-waves, etc. But the sensory nerve whose expan- 
sion in the sense-organ receives the stimulus from 
without, and transmits this stimulus to the central 
organ or brain, is in each separate sense-organ capable 
of only one special kind of perception. The optic nerve 
perceives light-waves alone, the auditory, sound-waves. 
In like manner the olfactory nerve deals only with 
olfactory sensations, the gustatory only with those of 
taste. The optic nerve can never perceive sounds, nor 
the auditory nerve colors. The skin cannot read a 
letter, nor the tongue listen to a symphony as the 
spiritualists, mesmerists and other rogues have asserted. 
On these facts the great biologist, Johannes Miiller, 
based his celebrated theory of the peculiar functioning 
of individual sensory nerves, of their specific energy. 

Full of meaning as is this theory of the specific 
energy of the sense-nerves, it is subject to an important 
limitation at the hands of our modern theory of deve- 



lopment. For in view of the embryological truth, that 
all the different sense-organs have their specific nerves 
developed from the integument, it must be admitted 
that the specific functions of the individual nerves of 
sense were not a primary quality of these latter, but 
have arisen as result of adaptation. Optic and auditory 
nerves, no less than olfactory and gustatory, were 
originally simple epidermal nerves, as they are to-day 
in the lower animals, and in the youngest embryos of 
the higher. At first all nerves of perception could only 
perceive simple changes of pressure and of temperature.. 
Then, by degrees, some of them learned to understand 
those influences that were brought to bear upon them 
by sapid and odorous bodies. Others entered upon a 
higher path, and passed on towards the recognition of 
sound-waves and light- waves. All the different sense- 
nerves have arisen originally by specialisation from 
simple epidermal nerves ; and in like fashion we must 
regard all the different sense-organs, which are essen- 
tially nothing else than collections of expanded nerve- 
ends, as local specialisations or differentiations of one 
universal sense-organ, the integument. The simple 
tactile sensibility of this integument, its power of per- 
ceiving changes of pressure and of temperature, forms- 
the starting-point for the specific energies of the higher 
nerves of sense. These last have evolved gradually, 
in the course of time, from the general sensibility of 
the outer layer of the body. 

This idea, as to the descent of the higher sense- 
organs and functions, receives an important extension 
if we descend among the lower animals still further, 
to those lowest organised forms that are sometimes 
named primitive animals, Infusoria, Protozoa, and are 
sometimes placed, as a special neutral kingdom called 
Protista, between the plants and animals. 

Among these remarkable Protista — of whom we shall 
only mention in this connexion the active Ciliata and 
Flagellata, the Rhizopoda, manifold in form, the im- 
portant Amoebae — we encounter sensory perception in 
various stages of development. The majority are sen- 
sitive, not only to variations of pressure and of tem- 
perature, but of light also. If a glass of water, in 



which many of these Protista are, is placed in the 
window, so that part of the vessel is in the light, part 
in the dark, most of the species present gather them- 
selves together on the exposed side, but some are turned 
round towards the dark side. Already among these 
microscopic Protista are there some that love light, and 
some that love darkness rather than light. Many seem 
also to have smell and taste, as they select their food 
with great care. 

Although such various kinds and degrees of sen- 
sory perception are easily and distinctly proved amongst 
these small creatures, none the less special sense-organs 
are Vholly wanting — -nay, nerves even are altogether 
absent in them. Here also we are met by the weighty 
fact, that sense-function is possible without sense- 
organ, without nerves. In place of these, sensitiveness 
is resident in that wondrous, structureless, albuminoid 
substance which, under the name of protoplasm or 
organic formative matter, is known as the general and 
essential basis of all life phenomena. 

In most of these low forms of animals the whole 
body has throughout its life only the structure of a 
single, simple cell. It consists only of protoplasm en- 
closing a nucleus. Either the w T hole structureless mass 
of protoplasm, or the most superficial layer of the 
protoplasm, a layer often clearly marked off from the 
rest, performs in these unicellular Protista the func- 
tions of sensation, and takes the place of the sense- 
organs that are wanting. Already in many the 
separation of such sense-organs has begun, when the 
protoplasm gives off from its surface fine threads, se ae 
or cilia. These are naturally exposed in an especial 
degree to the changes of impression that occur in the 
surrounding water, and are therefore better adapted 
for sensation than the rest of the surface of the unicel- 
lular body (Fig. 58). 

Whilst in these low animals or Protozoa, the simple 
cell can perform simultaneously all the life-functions, 
sensation, motion, digestion, reproduction, we find on 
the other hand that in all true animals (in all Metazoa) 
the body is of many cells, and its different functions 
are shared among several cell-groups. But in these 



beings also in every case the whole animal body con- 
sists at the beginning of its individual existence of a 
single cell only, the ovum. In many of the lower 
plant-like animals, e.g. the sponges, the ovum moves 
about independently, creeping like an Amoeba, within 

Fig. 58. 

A unicellular Infusorium of the order Ciliata (Prorodon). 

a. Oral aperture of the cell with funnel-shaped oesophagus. 

b. Contractile vesicle. c. Food-vacuoles in the sarcode. 
d. Nucleus. The whole surface of the cell is studded with 
fine hairs or cilia, that serve at once for sensation and for 
spontaneous movement. 

Fig. 59. 

Ovum of a calcareous Sponge (Olynthus), that moves and 
feels like an Amoeba. 



the body of the parent, and shows at that time evident 
power of sensation, contracting under contact or irri- 
tation (Fig. 59). 

But the original unicellular condition of the animal 
body after fertilisation of the ovum passes into a mul- 
ticellular. At the very beginning of embryonic de- 
velopment, the ovum, by repeated division, breaks up 
into many cells. The globular cell-mass thus formed 
is transformed into a hollow sphere, whose wall con- 
sists of a single cellular layer, and by invagination this 
hollow sphere gives rise to that remarkable embryonic 
form that we denote by the name of Gastrula (Fig. 60). 

Fig. 60. 

Gastrula of a mammal (rabbit). The whole body (shown in 
vertical section) consists of 96 cells, i.e., 64 clearer, smaller 
cells of the epidermis (e), and 32 darker, larger cells of the 
mucous ]ayer {i). The latter block up the gastric cavity (d) 
and oral opening (o) of the Gastrula. 

In all true animals or Metazoa, in the course of the 
advancing development of the individual, appears an 
embryonic form that can be referred to such a Gas- 
trula condition. This is, however, always wanting in 
the Protozoa. 

The cup-shaped, ovoid Gastrula (Fig. 61) contains a 
simple, hollow cavity, the digestive or gastric (g\ and 
this opens by a mouth that serves for the ingestion of 
food (p). The wall of this cavity is composed of two 



different cellular layers, the so-called primitive layers 
of the blastoderm. The inner, mucous layer, or endo- 
derm (i) effects the nutrition and assimilation of the 
body, and from it are formed the organs of digestion. 
The outer serous layer or ectoderm, on the other hand, 
(e) is of special interest to us at present. For the 
sensory cells that compose it, have to do with the 
knowledge of the external world, and as the integu- 
ment of the Gastrula represent a sense-organ in its 
simplest form. 

A . B 

Fig. 61. 

G astro 1 a of a calcareous sponge (Olynthus). A, seen from 
without B, in longitudinal section, e. External embryonic 
]ayer (epidermal layer or ectoderm). i. Inner layer 
(mucous layer or endoderm). g. Primitive alimentary 
canal or gastric cavity, o. Oral aperture. 

In all the Metazoa, not only do the cells that later on 
make up the skin, develop from this external layer, but 
the cells that form the nervous system and the rest of 
the sense-organs, have the same origin. Nerve-cells 
and sense-cells are alike, therefore, primarily deriva- 
tives of epidermal cells, and Remak was perfectly 
accurate when, thirty years ago, he spoke of the integu- 



mental covering of the embryo with its double wall as 
a sensory layer. 

Whilst most of the lower animals pass very rapidly 
through the Gastrula-stage in their development ; there 
are, at the present time, some lower animals that struc- 
turally are but little above this stage. Such permanent 
Gastrula-forms are met with in the Gastroeada (Physe- 
maria), the lowest sponges and the hydraform polyps. 
Amongst the last, the common fresh-water polyp or 
Hydra is of special interest. For although this little 
cup-shaped animal is very sensitive to touch and irrita- 
tion, to warmth and light, distinct sense-organs are as 
wanting in it as a nervous system. Individual cells of 
.the ectoderm attend to these functions. Already, how- 
ever, differences in sensitiveness of different ectoder- 
mal cells are evident. The delicate, tactile sensibility is 
especially localised in a circle of fine tentacles, sur- 
rounding the mouth and serving, at the same time, as 
prehensile organs for the seizing of the food. 

Such feelers, or tentacles, are met with generally in 
the lower animals, very widely distributed, in great 
variety, and often in great numbers. In many inverte- 
brate groups that are without eyes and ears, but are, 
nevertheless, sensitive to light and sound-waves, the 
epidermal cells of the tentacles seem to take the place 
of eyes and ears. Such are, e.g., the corals, Polyzoa, 
many Vermes. Very frequently, delicate, hair-like or 
bristle-like processes are seen on particular cells of the 
epidermis of the tentacles, and these hair-carrying cells 
that are often developed in other regions of the body 
we must clearly regard as having a special claim to the 
title of sense-organs. For not only are there among 
the lower aquatic animals cells of this kind, whose 
protoplasm is extended into a delicate process, out- 
stretched into the water and specially fitted to perceive 
alterations of pressure and changes of temperature, but 
these cells and processes appear well fitted to perceive 
distinct and regularly recurring oscillations of the 
water as sounds. It is, therefore, very probable that 
the widely distributed hair-carrying sensory cells, 
which we see in the epidermis of lower animals, have 
generally to do not only with sensations of touch and 



of temperature but with sound perceptions, that they 
are already incipient ears. And this idea is the more 
probable as the senses of touch and of hearing are 
generally very nearly allied, and as the first stages of 
development of true auditory organs are formed of like 
hair-carrying epidermal cells. 

The great difficulty, that we meet at the outset, of 
distinguishing simple tactile organs from the earliest 
stages of veritable auditory organs, is of deep interest. 
For in this very fact is evident the close relationship of 
the various sensations, and hence we can understand 
how the different senses could have been evolved from 
the lower, general power of feeling resident in the 
integument. A like difficulty meets us in the com- 
parative study of the other senses, and the same expla- 
nation tells in these cases also. 

For example, in regard to the two chemical sense- 
organs of taste and smell, we are not in the position to 
make definite statements as to their characteristic 
nature, as to the line of demarcation between them 
and organs of touch. 

We find, for example, on the proboscis of the fly, 
(Fig. 62) and on other parts of the mouth of insects, 
delicate sensory rods (s) projecting from the integu- 
ment. These rods or setae are in connexion with 
sensory cells (g) into which run branches of the sensory 
nerve (n). But we are unable to say with certainty 
whether these sensory rods are subservient to touch, 
taste, or smell, or whether, it may be, they have to do 
with blended sense-impressions. For taste and smell 
are very nearly allied to the simple touch, and only 
differ in essence from the latter in that the chemical 
influence of various bodies, perceived by the sense- 
cells of the particular organs in different manners, is 
translated into gustatory and odorous impressions. In 
the organ of taste the chemical changes are brought 
about by liquid substances that have been dissolved in 
water, in the organ of smell by gaseous matters, of 
infinite minuteness, borne by the air. At all events 
this is the case in the air-breathing Yertebrata, and we 
are accurately informed on this head in regard to these 
animals alone. It is, therefore, further, very doubtful 



if many organs that we look upon when they are met 
with in the lower aquatic animals, as the simplest 
organs of smell are not, in truth, rather organs of taste. 
It is as impossible to draw a sharp line of demarcation 
between taste and smell as between these two chemical 
senses and touch. 

Hence the views of zoologists on the distribution of 
the two chemical senses in the lower animals differ 
very widely. Many hold that taste and smell are very 

Fig. 62. 

A small fragment of integument from the proboscis of a fly 
(Musca) : vertical section. A sensory nerve (n) runs to the 
sensitive epidermis, whose cuticle is studded with fine hairs 
(c). The branches of the nerves run into groups of sensory 
cells (g) that end in projecting sensory rods (s). 

general and only rarely are wanting. Others think 
they are absent in the majority of the lower animals. 
This much is certain, that a great number of lower 
animals choose their food with as much care as a gour- 
mand ; as to insects, we know that some of them have 
an extraordinarily delicate sense of smell, and scent 



out odorous bodies at great distances. Yet special organs 
for the perception of sapid and odorous substances are 
generally unknown with any approach to certainty. 
Wherever we do recognise these definitely they are 
only different parts of the integument whose cells have 
become adapted to chemical sense-perceptions : cup- 
shaped cells for taste, rod-like ones for smell. Fre- 
quently special depressions are seen in the neighbor- 
hood of the mouth, wherein such gustatory and olfac- 
tory cells are placed. 

Even in the higher Vertebrata and in Man, in whom 
the organs of taste lie in the cavity of the mouth, the 
organs of smell in the cavity of the nose, the gustatory 
and olfactory cells are derivatives of the epidermal 
cells. The oral cavity, with the tongue and the palate, 
does not take origin along with the rest of the ali- 
mentary tract, but is derived from the epidermis, in 

Fig. 63. 

Gustatory cells from the tongue of a rabbit, a. Four separate 
cells connected inferiorly with very delicate terminal 
branches of the gustatory nerves, b. Two gustatory cells 
connected with an epithelial cell. 

the same way as the nasal cavity. Both originate from 
involutions of the integument. The gustatory cells of 
the tongue, the olfactory cells of the nose, arise also 
actually from the cells of the external, not from those 
of the internal, layer of the blastoderm. 

The gustatory or taste-cells (Fig. 63) are in man, as 
they are in other Mammalia, delicate, rod-like, or 



acicular cells, connected with the terminal fibres 
of the gustatory nerve, and invested by the broader 
epithelial cells (b) as a protective covering. They form 
numerous cup-shaped gustatory corpuscles (Fig. 64) or 
taste-bulbs scattered upon the surface of the tongue. 
In the centre of each corpuscle lies a bundle of gus- 
tatory cells, surrounded by investing epithelial cells. 
When food placed on the tongue comes into contact with 
the gustatory corpuscle, the sensation of taste results 
through the agency of the gustatory cells. 

Fig. 64 

B. Four tactile corpuscles from the tongue of a rabbit. Only 
the two middle ones are complete. Vertical section through 
the surface of the tongue. 

The olfactory cells in the mucous membrane of the 
nose are very like the gustatory cells of the tongue. 
They also are very delicate slender cells placed vertically 
in the epithelial surface, having their inner ends in 
direct connexion with very fine terminal fibrils of the 
olfactory nerve (Figs. 65, 66). As a rule, in the nasal 
mucous membrane of Vertebrata two kinds of olfactory 
cells are distinguishable, that in all probability have 
different offices in respect to the perception of odors. 
The more slendrer ones, often filiform acicular cells, 
are markedly swollen in the middle, where the nucleus 
lies (Fig. 65 e), and carry in the Amphibia a tuft of 
exceedingly fine and thin olfactory cilia at their free 
ends. The broader, rod-like, or cylindrical olfactory 
cells on the other hand (Fig. 65, a, b) carry no such 



cilia, and are regarded by many as simple epithelial 
cells. The nasal mucous membrane, in which these 
cells are placed, lines in the higher Vertebrata, as in 
Man, the internal wall of the nasal cavity. But it also 
is originally a part of the external integument. For 

Fig. 65. Fig. 66. 

Three olfactory cells from the nose of an Amphibian (Pro- 
teas). In the middle a larger cylindrical cell (a, without 
cilia ; on each side of it a filiform olfactory cell, swollen in 
the middle into a globular shape, and bearing a tuft of very 
delicate olfactory cilia at the free extremity. 

Five olfactory cells from Man, three more delicate, rod-like, 
and between these, two broader, cylindrical ; all are con- 
nected below with terminal fibrils of the olfactory nerve. 

the nose begins, even in these higher animals,- in the 
same fashion that is persistent in Pisces, their lives 
through, as a pair of depressions of the integument. 



Only in the course of embryological development these 
olfactory pits (Figs. 52 — 53 ri) gradually retreat into the 
interior of the body, and in like manner in the course 
of evolution they have undergone the same change of 
place. The olfactory cells of the nose are, like the 
gustatory of the tongue, historical derivatives of the 
general tactile cells of the external integument. 

If the ordinary distinction between the lower and 
higher sense-organs is maintained, the latter title only 
belongs in reality to those two noblest and most 
wonderful organs of the animal body that we call ear 
and eye. For the organ of hearing and the organ of 
sight alone reach that marvellous perfection of delicate 
structure, with its related complex division of labor, 
that makes them the most valuable instruments of our 
soul-life. These organs alone are the aesthetic sense- 
organs, the invaluable psychical instruments opening 
to us the portals to our highest blessings, Art and 

Fig. 67. 

B. Auditory cells from the cochlea of the oar of a dove (Co- 
lumba). a, b, c. Three separate ciliated cells, a and b 
seen in profile, c. The free end. a Bundle of auditory 
cilia, b. Clear cup-shaped space, c. Nucleus, with nucleo- 
lus d. Dark thread (probably continuous with a very 
delicate terminal nerve-fibre), d A ciliated cell (e) in con- 
nexion with a pointed cell (/) that has peculiar clavate 
appendages (g). e. A tegumental cell with dark internal 
part (m) and clear external part (n). 




Whilst, then, the lower sense-organs that serve for 
the perception of pressure, temperature, taste, smell, 
exhibit generally throughout the animal kingdom 
simple uniform arrangements, in the higher sense- 
o-gans of hearing and sight, on the other hand, we 
meet with a number of complex and varying arrange- 
ments that awaken our deepest astonishment. Never- 
theless, even here again, the real workers in perception 
are but highly developed cells. These esthetic cells, 
the auditory of the ear, the visual of the eye, are m 


B. A Medusa (Eucope). From the middle of the bell-shapedi 
body depends the gastric cavity, whence four digestive 
canals pass to the margin of the disk. The ova (g) lie in 
these canals. From the circumference of the disk (6) dj 
pend four prehensile tentacles, and between these are eigm 
auditory vesicles (a). 



their primal origin nothing but cells of the integument 
transformed and specialised. 

The wonderful auditory cells of the ear are adapted 
to their special function, that of the perception of 
sound, in that they carry delicate bristle-like processes, 
the auditory cilia (Fig. 67, auditory cells from the ear 
of the dove). 

Hence these also are called (not very conveniently), 
hair-cells. Sometimes each auditory or hair-cell carries 
only one delicate cilium, sometimes a whole bundle or 
tuft of them. The sound-waves brought to the animal 
via the water or the air, strike upon these auditory cells 
and cause their cilia to vibrate. In many other animals, 
e.g., Polyps, Medusae, Vermes, the like individual 

Fig. 69. 

Soul-apparatus of a Pteropod (Firola or PLerotrachea. a. 
Auditory vesicles, g. Brain, c. Nerves (circumoesopha- 
geal). o. Eyes. I. Lens. ch. Pigment-membrane of the 
eye. r. Expansion of the optic nerve. 

auditory cells are distributed throughout the integu- 
ment irregularly or in special regions. But in the 
majority of the lower animals the auditory cells consist 
of two globular vesicles that lie usually in the vicinity 
of the nervous centres, sometimes deeply placed within, 
sometimes close to the surface of the skin. Many 
Medusas have numerous auditory vesicles, lying quite 
freely in the margin of the disc : Eucope, .e.g., has 
eight (Fig. 68). These auditory vesicles (Fig. 69 a) are 
filled with liquid or gelatinous matter, and their wall 
is Ikied internally with a layer of cells, carrving in 




whole or part delicate cilia, and thus proving that they 
are auditory cells (Fig. 70 e). 

From without an auditory nerve runs to the vesicle 
(Fig. 69), and gives off to the individual cells its finest 
fibrils. In the middle of the vesicle swims, as a rule, 
an otolith (Fig. 70, o) or nucleus of calcareous matter, 
or a concretion of many crystals of calcium carbonate. 
The delicate ends of the auditory hairs, or fine cilia of 
the auditory cells, seem for the most part to come into 
contact with the surface of the otolith. 

Fig. 70. 

Auditory vesicle of a mussel (Cyclas). c. External capsule. 
e. Auditory cells, with cilia, o. Otolith. 

The vibrations of the sound-waves that are trans- 
mitted from the exterior through the wall of the 
vesicle, pass through this wall to the fluid within, and 
to the otolith swimming in it. The auditory cilia re- 
ceive the sound-waves collected here, and translate 
them into the perception of the noises, or of the note, 
that is now borne by the auditory nerve to the nerve- 

In a great number of Vermes, in the Ascidioida, 
Mussels, Snails, Brachiopoda, Crustacea, we meet with 
the auditory organs in this simple form, as globular, 
closed auditory vesicles, containing fluid and in the 
midst an otolith. But in the Crustacea are many ani- 
mals, among others our ordinary crayfish and lobster, 
that have the auditory vesicles not enclosed, but con- 
nected by a short passage with the external skin, and 
directly communicating with the surrounding water. 
Instead of the usual calcareous otoliths that are formed 
by the animal itself, in these Crustacea there are little 
silicious or arenaceous particles that have been taken 
in from without. Nevertheless the sense of hearing is 



in these animals very well developed, and many delic- 
ate cilia, on the inner surface of the auditory organ, 
serve for the perception and distinction of the various 
sounds. If we give rise, by playing the violin, to notes 
of varying pitch, and at the same time observe the 
auditory organ under the microscope, we see that at 
each note only a particular auditory hair is set in 
vibration. There is present a suitable keyboard for 
notes, of such a nature that the number of undulations 
of every note corresponds with a cilium of definite 

These facts are of the deepest interest in many ways, 
and especially in that they point us to the origin of the 
internal auditory vesicle from the integument. The 
auditory vesicles arise in the surface of the skin as 
shallow depressions, lined with ciliated cells. These 
depressions gradually deepen, fashion themselves into 
auditory organs, and, cutting themselves off entirely 
from the skin, become closed auditory vesicles. In the 
Medusae also, as in the Crustacea, the phylogenetic 
origin of the auditory vesicle is established by the 
comparison of the stages of development that follow 
so closely one on the other, and it is completely settled 
by embryological investigations. In many Medusae 
there are short tentacles that become transformed 
directly into auditory vesicles. They will coil up and 
lie as auditory bulbs within a vesicle. The auditory 
cilia within the latter, that are responsive to sound- 
waves, were earlier simple tactile hairs belonging to the 
epidermal cells, and then could only recognise altera- 
tions of pressure. They have gradually become adapted 
to the perception of the more rapid sonorous vibra- 

Here again we see how difficult is the distinction 
between auditory and tactile organs. For we cannot 
tell as we study the delicate auditory cilia under the 
microscope, whether they only respond to alterations of 
pressure or have already learned to perceive sonorous 
vibrations. But it is even more remarkable that in 
many lower animals, especially in the Articulata, that 
clearly have the sense of hearing, we are not able at 
present to distinguish special organs for that sense. But 



we find in these articulate animals sense-cells that 
carry hairs, and are connected with the nerves of the 
skin, widely distributed through the skin ; and, as 
their firm and elastic exoskeleton is excellently adapted 
for transmission of sound-waves, it is very probable 
that different parts of the external covering function as 
auditory organs. This conjecture is strengthened by the 
fact that perfect auditory organs are met with in the 
Articulata in very different parts of the body. Whilst 
in our common lobster and crayfish they lie in the head at 
the base of the internal feelers or antennules, in other 
crabs, on the contrary, as Mysis, we find them thrown 
backwards to the tail. The auditory organs in the 
musical Orthoptera lie at times at the side of the thorax, 
as in the well-known migratory crickets (Acridina), at 
times in the tibia of the front leg, as in the cricket and 
the green grasshoppers, in Gryllus and Locusta. It is 
beyond a doubt that these various organs of hearing 
in various places, having no relation one with another, 
have originated from the integument. For if they 
were inherited from some common ancestor, they 
would be situated in corresponding or homologous 
parts of the body. 

The organ of hearing rises to a yet higher stage of 
development among the Vertebrata, although the 
arrangement of the parts follows in all essentials that 
met with in the lower forms. With the solitary excep- 
tion of the lowest vertebrate, the well-known Lancelet 
or Amphioxus, we have in all Vertebrata, from the 
Pisces up to Man, a pair of conspicuous auditory 
vesicles located in the head. Each consists of two parts, 
an upper, the vestibule, and a lower, the utricle. In 
each part lies an otolith, or a collection of conjoined 
calcareous crystals, near which the auditory nerve 
spreads out on the inner wall of the vesicle, its finest 
fibrillae coming into connexion with the auditory cilia 
placed thereon. From the upper — vestibule — pass off 
generally three annular or semicircular canals ; their i 
cavities are in connexion with that of the vestibule, j 
and, like it, are filled with fluid endolymph (Fig. 71). j 

From the lower — utricle — in the higher Vertebrata, is ! 
deyeloped a peculiar organ that has been named the rJ 



cochlea, on account of its outward resemblance to the 
shell of a snail. It would seem that this cochlea has 
to do with musical perception alone, whilst the vestibule 
and its derivatives are concerned in the perception of 

The delicate construction of this internal auditory 
organ is so extraordinarily complex in man and in the 
higher Vertebrata, that the name of labyrinth has rightly 

Fig. 71. 

Auditory vesicles (the so-called membranous labyrinth) of 
different Vertebrata. a, Man ; b, Calf ; c, Pike ; d, Hawk ; 
e, Frog. 1, 2, 3, the three semicircular canals (1, horizontal ; 
2, superior ; 3, posterior). 4 Part common to the superior 
and posterior canals. 5. Ampullae (enlargements). 6. Ves- 
tibule. 7. Utricle. 


enough been given to it. Yet the astounding complexity 
of structure presented by this labyrinth, to the outlet 
from whose mazes the Ariadne-threads of embryology 
alone can be our guide, is primarily nothing more than 
a simple auditory vesicle, and, like the simple auditory 
vesicles of the lower animals, has been developed from 
the integument. This notable discovery was made in 
1831 by Emil Huschke, in Jena. In order to convince 
ourselves as to its accuracy we have only to examine a 
fowl's egg that has lain for a day and a half in the 
hatching machine. In it we see on the side of the 
rudimentary head of the embryo chick a pair of shallow 
depressions lined by cells of the integument. By the 
third day of incubation these have become deep auditory 
cavities, that communicate with the exterior only by a 
narrow passage (Figs. 52, 53, g, p. 203). By the end of the 
third day they are cut off completely from the skin (Fig. 
72, A, B). On the fourth day the isolated rounded auditory 

Development of the auditory vesicle or labyrinth of the ear 
in an incubated chick (in five successive stages. A — b). Ver- 
tical, transverse section of the rudimentary head. fl. Audi- 
tory depression. Iv. Auditory vesicle, h. Appendage of the 
labyrinth, c Rudimentary cochlea, csp. Posterior semi- 
circular canal, cse. External semicircular canal, jv. Jugular 

vesicle has already receded some distance into the head, 
and soon it becomes constricted in the middle, so as to 
mark off the vestibule above from the utricle below 

Otoliths appear in both divisions. From the vesti- 

Fig. 72. 

(Fig. 72, C, D). 



bule arise the three semi-circular canals, from the 
utricle, the cochlea (Fig. 72, E). Thus all the chief 
parts of the labyrinth are formed, and gradually acquire 
their delicate and perfect structure. But the most 
delicate auditory cells, that are developed later on in 

A piece of the organ of Corti or the lining membrane of the 
cochlea of a dog. a. Crista spiralis, b. Inner cell layer 
(epithelium of the sulcus spiralis), c. Pillar heads of the 
fibres of Corti. D. Lamina reticularis with outer ciliated 
cells. E. Outer cell layer (epithelium of the membrana 
basilaris). a. Cells of the sulcus spiralis, b. Outer bound- 
ary of the acoustic teeth, c, q. Meshwork between the lin- 



ing cells, d. Yas spirali. e. Inner ciliated cells. /. Inner 
pillars (fibres of Corti). g. Limit between / and h. h. Outer 
pillars. p. Three rows of external ciliated cells, n. Sup- 
porting cells. (Highly magnified). 

the cochlea, are still originally none other than descend- 
ants of ordinary epidermal cells. Here, again, the history 
of the embryo is a condensed epitome of that of the 
race. In the same way as in a few days the labyrinth 
of the ear of the chick is developed out of the integu- 
ment, so in the course of many millions of years has 
the marvellous structure of our human auditory laby- 
rinth been evolved out of the simple ear-vesicle of the 
lower animals. 

The part of the labyrinth in man, and the other 
higher Vertebrata that surpasses all others in the won- 
drous delicacy and complexity of its structure, is the 
so-called organ of Corti, or the lining membrane of the 
cochlea (membrana tectoria cochleae, Fig 73). 

This wondrous organ is, when compared with the 
simple ear-vesicle of lower animals (Fig. 67), as a piano 
of the best makers with its keys in comparison with 
the simple vibrating cord or string that an Indian has 
strung upon his bow. In the canal of the cochlea we 
find a tunnel-like passage that is vaulted over by a 
series of delicate, bony arches, the fibres of Corti (C). 
Each fibre consists of an inner (/), and an outer pillar 
(A). On these fibres of Corti rest the most important 
acoustic constituents of the cochlea, the musical ciliated 
cells, set with very fine tufts, in which cells the very 
delicate end fibrils of the auditory nerves terminate. 
On the heads of the internal pillars (/) rests but a 
single row of ciliated cells (e) : on the heads of the 
outer pillars (h) 3 — 5 rows of external ciliated cells 
p). It is probable that the number and arrangement 
of these ciliated cells condition the musical capabilities 
of the different Mammalia. The musically cultured 
man seems to have 4 — 5 rows, the rude savage 3 — 4, the 
ordinary mammal only 3 rows ; the Wagnerian musi- 
cian of the future will probably have 6, or even a larger 
number. The very highly developed composition and 
arrangement of the cells in the organ of Corti, call to 



mind the like conditions in the retina of the eye. And 
as the latter has been gradually evolved in the course 
of many millions of years, from a simple layer of 
visual cells, so as the former from a simple layer of 
auditory cells. Visual and auditory cells alike have 
arisen from ordinary epidermal cells, and have gradu- 
ally withdrawn themselves from the surface into the 
sheltered interior of the body. 

But the totality of the acoustic apparatus in man, and 
in the higher Vertebrata, is by no means completed by 
the building up of this wonderful labyrinth. Other 
external parts become associated with this essential 
constituent of the auditory organ, parts that catch and 
conduct to the labyrinth sound-waves. These parts are 
wanting in Pisces. In these water-dwellers the sound- 
waves, in the water, strike directly on the skin and 
cranial bones, and through these reach the labyrinth 
lying within the skull. In many Pisces the percep- 
tion of sound is aided by the labyrinth being in con- 
nexion with the swim -bladder that is filled with 
air. In the herring this is effected through the 
medium of special air-canals, in the carp and shad 
through a chain of auditory ossicles. The hydrostatic 
apparatus of the swim-bladder serves, therefore, as a 

In the Amphibia (the salamanders and frogs), a special 
apparatus having to do with the conduction of sound is 
developed. To these animals, living alternately in 
water and on land, such an apparatus is of great service, 
as the air is not well- adapted for the conduction of 
sounds. A circular membrane like the head of a drum 
that lies in the skin covering the head, and receives 
the sound-waves in the air, is the boundary of a cavity 
filled with air and opening by a tube, the Eustachian 
tube, into the oesophagus. The labyrinth lies inter- 
nally to this tympanic cavity, and receives the sound- 
waves partly through the medium of the air it encloses, 
partly through the columella, a rod-like bone that puts 
the membrane of the tympanum into direct connexion 
with the wall of the labyrinth. The whole of this con- 
ducting apparatus, which the Amphibia have trans- 
mitted onwards to the higher Vertebrata, has been 



evolved from the first gill-cleft, and the two gill-arches 
that bound it in the fish. This is proved by compara- 
tive anatomy, and also by embryology. 

The external ear, which man has in common with 
the other Mammalia, is an evolution product of much 
later time. 

This external ear consists of the pinna (Fig. 74 a), 
which catches the sound-waves of the air, and the 
external auditory canal (b) that conducts them to the 

Fig. 74. 

Human ear (left) seen from the front, a. Pinna, b. External 
auditory canal, c. Membra na tympani. d. Tympanic cavity. 
e. Eustachian tube. /, g, h. The three ossicles (/. Malleus, 
g. Incus, h. Stapes), i. Vestibule, k. The three semi-circular 
canals. Z. Utricle, m. Cochlea, n. Auditory nerve. 

drum or membrane of the tympanum (c). These are 
developed from an annular fold of the epidermis that 
forms the boundary of the external part of the first 
gill-cleft. In the Mammalia that have a very acute 
sense of hearing far surpassing that of man, the pinna 



is much more completely developed and has much 
freedom of movement. Its direction, as well as its 
shape, is altered by special muscles, so that the sound- 
waves coming in different directions may be caught in 
the best possible way. The external ears of the inhabi- 
tants of wildernesses, the jerboas, the foxes of the 
Sahara, are of remarkable size and mobility. For it is 
of importance to these to catch the slightest sound from 
afar in the dead silence of the vast plain. On the other 
hand, in man, who is far inferior to these animals in 
quickness and delicacy of hearing as of smelling, the 
pinna has lost its importance, and has sunk to the level 
of a useless or rudimentary organ. Men whose ears 
have been cut off hear as well as they did before. 
Moreover, in many domestic animals whose ears hang 
loosely, dogs, rabbits, goats, the disuse of the muscles 
of the ear resulting from the domestication of the 
animals has led to their degeneration, and in these cases 
also the pinna has gradually become superfluous and 
useless. That the human pinna is a rudimentary organ, 
is demonstrated by the extraordinary variations in its 
size and shape. In these it surpasses, probably, all 
other organs. In large gatherings, when we are not 
deeply interested with the matter in hand, there is no 
amusement more instructive than an investigation of 
the endless varieties of pinnae to be seen. 

The Australian negroes, Papuans and other savages, 
whose acuteness of hearing is far beyond that of civi- 
lised races can, as a rule, control the movements of the 
pinna very completely. Many favored individuals also 
among the civilised peoples are at the present time 
capable of these movements, and some celebrated 
physiologists, e.g., Johannes Miiller, have only by 
energetic and long continued efforts of the will, and by 
practice continued through many years, arrived at 
the power of moving their ears freely and quickly. 
Herein we have one of the most notable examples of 
the great force of use and habit, of the mighty power 
of adaptation. For in these cases ancient, long disused 
muscles are brought again into active service by 
repeated nervous activity, by the force of persistent 



In other connexions the development of our auditory 
organ gives us very instructive information as to the 
astonishing power of use and habit, of education and 
adaptation. What a world of difference is there between 
the rude perception of sounds of a savage, whose high- 
est musical pleasure is in the rhythmical repetition of 
some noise, or at most of some simple sound of drum 
or of fife, and the musical perceptions of a cultured 
man, whose ear takes delight in the classic harmonies 
of an opera of Mozart, or a symphony of Beethoven ! 
What a vaster world of difference between these last, 
and the hyper-cultured folk that revel in the Wag- 
nerian music of the future, and find the true end and 
aim of aesthetic enjoyment of sound in complicated 
discords ! 

As every organic function evolves hand in hand with 
its organ, there is no doubt that with this historic 
advance in the power of perception of sound, is closely 
knit a like advance in the perfection of our auditory 
labyrinth. The minute structure of our cochlea is, to- 
day, different from that of our wild forefathers five 
thousand years ago. The labyrinth, moreover, of the 
wild savage of to-day in its minute structure, would 
probably present certain differences from that of the 
civilised man. It is no contradiction to this, that the 
former has a keener sense of hearing than the latter. 
For the acuteness of the ear in the savages, who hear 
sounds at a great distance, is altogether another thing 
than the delicacy of the musical ear of the cultured 
races. The ability of the one, stronger quantitatively, 
is very different from the ability of the other, which is 
higher in quality. This holds also in regard to the 
sense of smell and of sight. Whilst savages see much 
farther, and can recognise much more certainly weak 
odors than the civilised races, they, nevertheless, are 
far inferior to these in delicate discrimination of odors, 
and in the aesthetic perfection of the sense of color and 
of form, the result of thousands of years of civilised 

We find in the organ of sight exactly the same 
relationships between its historic and its gradual per- 
fection as in the ear. The eye, highest, most perfect of 



all the sense-organs, has not been suddenly called into 
being by the fiat of a designing creator, but, like all 
other organs, has slowly, gradually evolved by Natural 
Selection in the struggle for existence. As the eye and 
ear, the two noblest organs of the senses, the organs 
that perceive beauty, are very different, and yet com- 
parable in their anatomical structure and their physio- 
logical function, so is it also with their evolution. As 
the sense of hearing of the ear has grown out of the 
tactile sensibility of the skin, so has the sense of light 
of the eye grown out of the thermal sensibility of the 
skin. Comparative anatomy and embryology show us 
in relation to the eye, as to the ear, a long chain of 
stages of evolution. Here also we must come to the 
conclusion that the marvellous organ of sight of man 
and of the higher animals is but the final result of a 
long series of advancing adaptations that have been 
gradually accumulated by heredity, and that lead us 
step by step from the low T est stages to the highest. 

The first beginning of the organ of sight in the lower 
animals is no other than a simple dark spot in the clear 
integument, generally a black or red pigment-spot. 
Even in the unicellular Protista such dark specks of 
color-matter seem to serve for the perception of light. 
Single pigment-cells, or collections of such pigment- 
cells, constitute the beginning of a very simple eye in 
many Coelenterata and Vermes. When pigment is de- 
posited in the colorless integument of such animals, 
these dark spots must be more responsive to the changes 
of temperature in the surrounding water or air than the 
neighboring colorless regions of the integument. For 
it is well known that the light and heat rays are 
absorbed by dark bodies and reflected by colorless ones. 
A black stone becomes hot more rapidly than a white 
one when placed in the sunshine. Hence with the 
formation of dark specks in the integument there is the 
first beginning of an eye, but clearly only of a tem- 
perature, or a light-eye that can distinguish between 
warmth and cold, clearness and darkness, better than 
the rest of the skin around it. The ordinary nerves of 
the skin that pass to these dark pigment-cells of the 
integument have already trodden the first steps of that 



magnificent march, at whose end they have developed 
into the highest nerves of sense, the optic nerves. 

But we expect more from a veritable eye than simple 
distinguishing between clearness and darkness. The 
true eye gives a picture of the conditions of the en- 
vironing outer world ; and the inner lining of the eye 
on which this picture is painted, as on the sensitive 
plate of a photographic apparatus or in a magic-lantern, 
is the expansion of the optic nerve, the retina. But a 
picture of this kind can only be formed on this sensi- 
tive nerve-layer when a refracting body, a lens, is 
present. This curved lens, like a burning-glass or a 
simple magnifying-glass, collects the rays of light 
coming from external objects, and throws on the retina 
a diminished image of those objects, that is felt by the 
optical cells and transmitted by the optic nerve to the 
brain. By the formation of a transparent refractive 
lens, the crystalline, the simple light-eye makes a great 
step onwards, and becomes a true picture-eye. This 
immensely important advance is accomplished in the 
lower animals, especially in Coelenterata and Vermes, 
in very different ways. Sometimes a single epidermal 
cell, much expanded, globular or lenticular, curved, 
develops into the crystalline lens. Sometimes a group 
of conjoined cells, sometimes a hardened distinct part 
of the skin (as the chitinous lens of the Arthropoda), 
thus develops. 

We have already all the materials for the building up 
of the very complex eye of the higher animals and of 
man, viz. : 1, a refracting lens, situated in the integu- 
ment ; 2, the optic nerve, spreading out on the inner 
surface of the lens as the retina ; 3, a pigment layer, a 
layer of dark integumental cells, enveloping at once the 
retina and the lens. The crystalline lens refracts the 
rays of light and blends them into a picture ; the pig- 
ment layer absorbs them, and the optical cells of the 
retina translate them into sensation, that is carried by 
the optic nerve to the central apparatus of the brain. 
All these essential parts of a simple eye are originally 
formed from the epidermis ; a fact proved by com- 
parative anatomy and embryology alike. Hence we 
draw this most weighty conclusion in regard to the 



ancestral history of animals that the slow gradual 
evolution of the organ of sight in the ages has, in the 
course of many millions of years, passed along the same 
way, and that in all cases the eye has been originally 
developed from the cells of the epidermis. 

It is clear that it is a long journey from this simple 
eye of the lower animals with its three essential parts 
to the far more perfect eye of the higher animals, that 
may be composed of more than thirty different parts. 
Interesting as it would be to follow the long march of 
advancing evolution step by step, it is impossible in 
this place to enter more fully into the question, on 
account of the perplexing complexity of the minute 
anatomical and genetic relations of the parts. The 
wondrous structure of the eye in the various classes of 
animals is far more variable and more perfect than is 
that of the ear. And as the sense of sight is far higher 
than that of hearing, as the art of painting at its best 
is in advance of that of music, so also the structure and 
evolution of the eye are in like manner more fasci- 
nating, more wonderful than that of the ear, and, it 
must be confessed, more difficult. We must, therefore, 
content ourselves in this connexion with some brief 
statements as to the more general details of the evolu- 
tion of the organ of sight. 

In the first place, Natural Selection in the advancing 
structure of the eye improves the apparatus that refracts 
the light, substituting for one simple lens a very com- 
plex combination of variously refracting bodies, of 
which the most important are a hard laminated lens 
and a white semi-fluid vitreous body (Fig. 75, I, h). 
By this means the optical errors of the single lens are 
avoided. Next, in the place of a simple pigment layer, 
we meet with a differentiated vascular coat, the choroid, 
of many layers, with tapetum, fringes, etc. Lastly, 
and most important of all, the nervous apparatus of the 
eye is rendered more perfect to an astounding degree. 
In place of a simple nerve-expansion we find a very 
complex retinal structure, composed of many different 

The great group, Vermes, is in an especial degree 
instructive in the study of this evolution of the eye in 



time, for in that group we can follow out a complete 
series of gradations in the structure of that organ. In 
the lowest Vermes the eye is only made up of indivi- 
dual pigment-cells. In others, refractive cells are asso- 
ciated with these, and form a very simple lens. Behind 
these lens-cells optic-cells are developed, forming a 
single-layered retina of the simplest order, whilst they 
are in connexion with the very delicate terminal fibrils 
of the optic nerve. Finally, in the Alciopidaa very 
highly organised Annelidse, that swim on the sur- 
face of the sea, adaptation to this mode of life has 
conditioned such perfection of the eye that this organ 

Eye of an Annelid (Alciope). I. Lens. h. Vitreous humor. 
b. Rod and cell layer, jp. Pigment layer, o. Optic nerve, 
o'. Expansion of the last. i. Integument, forming a horny 
layer in front of the eye (cornea, c). 

in these animals is in no way inferior to that of the 
lower Vertebrata (Fig. 75). In these beings we find a 
large globular eyeball enclosing externally a laminated 
globular lens (I), internally a vitreous body (h), of large 
circumference. Immediately investing these are the 
rods of the optical cells (b) sensitive to light, that are 
separated by a layer of pigment-cells (p) from the outer 



Fig. 75. 



expansion of the optic nerve (o), the retina (V). The 
external epidermis (i) invests the whole of the pro- 
minent eyeball, and forms in front of it a transparent 
horny layer, the cornea (c). 

If we compare the highly developed eye of this worm 
with that of man (Fig. 76) or of any other of the higher 
Vertebrata, we find in all essential details a like arrange- 
ment of parts. Only in man the cornea (b) is more 
strongly convex, and the lens (I), by consequence, less 

Fig. 76. 

Human eye, cross section, a. Sclerotic, b. Cornea, c. Con- 
junctiva, cl. Circular vein of the iris. e. Choroid. /. Ciliary 
muscle, g. Corona ciliaris. h. Iris. i. Optic nerve, h. 
Anterior limiting membrane of the retina. I. Crystalline 
lens. m. Membrana Descemetica. n. Pigment layer, o. 
Retina, p. Canal of Petit, q. Macula lutea. 

globular and more flattened in shape. A very vascular 
choroid (e) lies internally to the strong outer sclerotic 
(a) and forms anteriorly the colored iris (7i), which in 
its turn surrounds the pupil. Between the choroid (e) 

z 2 



and retina (o) lies a simple layer of very regular hexa- 
gonal pigment-cells, filled with black pigment (Fig. 
77, a). 

This dark tapetum or pigment layer belongs both by 
origin and by its optical significance to the retina. 

The most important and most remarkable part of the 
eye in man, as in other animals, is the retina, a very 
delicate thin membrane, fermed essentially of optical 
cells. These cells are connected with the very fine 

Fig. 77. 

The pigment layer, a. Ten cells, surface view. b. Two cells, 
side view. c. One with rods attached to it. 

end-fibrils of the optic nerve, and are, like most other 
sense-cells, slender and rod-shaped. In the lower ani- 
mals these optical rod-cells are of simple and uniform 
nature. In the higher animals they are distinguished 
as Jwo differently-shaped structures, called rods and 
cones (Fig. 78). 

The rod-cells are longer and thinner, the cone-cells 
shorter and thicker. The former, which carry externally 
a delicate crystalline rod, seem to aid in the perception 
of the form, whilst the cone-cells, which carry a pointed 
conical process, seem to aid in the perception of the 
color of the retinal pictures. Hence the lower animals, 
whose optic nerve-fibres end wholly in rod-cells, see 
colorless pictures only, and know, probably, nothing of 
color. Only those higher animals that have cones 
intercalated with the rods seem able to distinguish 
colors. In bats and other nocturnal animals we find in 
the retina only a few cones, or even none at all. But 
the cones are more numerous and better developed in 



the lizards and the birds that love the sunlight and 
have clearly a highly developed sense of color. 

In man, as in other higher Vertebrata, we can dis- 
tinguish in the retina not less than ten distinct layers 
(Fig. 79). 

Most externally lies the layer of black pigment-cells 
(pigmentosa, 10), and immediately beneath this is the 

Fig. 78. 

Nine optical cells from the posterior part of the human retina; 
three shorter thicker cone-cells lie between six longer thinner 
rod-cells. > 

layer of optic-cells with their rods and cones (7 — 9). 
This is separated from a thick layer of granular cells 
(5) by a thin granular layer (6), and the thick granular 
stratum is separated by another exceedingly broad 



granular layer (4) from a series of ganglion-cells (3) 
that are in direct connexion with the fibres of the optic 

Fig. 79. 

Vertical section through a piece of the human retina. 1. Mem. 
branalimitans interna. 2. Optic nerve-fibres. 3. Ganglion 
cells 4. Internal granular layer. 5. Intergranular layer. 
6. External granular layer. 7. Optic cells. 8. Memhrana 
limitans externa. 9. Eods and cones. 10. Pigment layer. 
Very highly magnified. 



nerve (2). The complication of structure and arrange- 
ment in the retinal elements corresponds with the stages 
of optical evolution reached by the eyes, so that the 
retina of the skilled painter will be more perfect than 
that of the rude savage. 

With these the most important parts, optically con- 
sidered, of the eye are associated in man, as in all the 
higher animals, many auxiliary structures, wanting in 
the older lower animals. Such are the intrinsic muscles 
of the eye that alter its form and adapt the lens to 
different distances ; the extrinsic muscles of the eye 
that move the organs as a whole in different directions. 
Around the eyeball is placed a firm outer membrane, 
the sclerotic, in which in many cases {e.g. in birds) a 
circle of bony plates is present. In front the sclerotic 
passes into a transparent membrane, the cornea. The 
lachrymal apparatus, that keeps the external surface of 
the eyeball smooth and clean, is only developed in the 
three higher vertebrate classes ; Reptilia, Aves, Mam- 
malia. The eyelids, however, that act as protective 
curtains and keep out impurities from the outer surface 
of the organ, have already appeared in Pisces, and have 
l>een transmitted from these to the higher Vertebrata. 

Not less interesting than these manifold advances that 
are evident in the structure of the eye in many classes 
of Animalia, are the retrogressions. Deeply placed 
within the head, covered by thick skin and muscles, 
true eyes, that cannot see, are found in certain animals 
that belong to various classes. Among the Vertebrata 
there are blind moles and fieldmice, blind snakes and 
lizards, blind Amphibia and Pisces. Amongst the 
Arthropoda we know many blind beetles and Crus- 
tacea. All these blind animals have grown accustomed 
to live in darkness. They shun the daylight, dwelling 
in holes or burrows under the ground. Thus they have 
lost the habit of seeing, and by the disuse of the organ 
the organ itself has become atrophied. All the animals 
known to us that live in this fashion were not originally 
blind, but have evolved from ancestors that lived in 
the light and had well-developed eyes. The atrophied 
■eye beneath the opaque skin may be found in these 
l)lind beings in every stage of reversion. In the higher 



Crustacea, whose eyes are placed on long freely mobile 
stalks, there are some blind dwellers in the earth (near 
allies of our river crayfish) in whom the ey© itself 
has vanished, but the eye-stalk is still present. As 
Darwin puts it in his trenchant style, we have here 
the stand of the telescope left, but the telescope itself 
is lost. 

Rudimentary eyes, such as these that see not, like 
the many other facts in the history of the evolution of 
the sense-organs, demonstrate in the clearest way that 
the most highly perfected sense-organs are not the 
artificial product of a thought-out plan of creation, but> 
like all other organs of the animal body, they are the 
necessary outcome of Natural Selection in the struggle 
for existence. On behalf of this mechanical or Monistic 
theory as to the origin of the senses the most convinc- 
ing utterance is given by the fact that occasionally in 
different animals eyes are developed on parts of the 
body that never before bore such organs. Thus the 
higher Mollusca, the Sepia and the snail, have always 
only one pair of eyes in the head, as in the Vertebrata. 
But in some Gasteropoda, as Onchidium, in addition, 
eyes in large number are developed on the back, and, 
most remarkable of all, the structure of these dorsal 
eyes does not resemble that of the cephalic eyes of the 
Gasteropoda, but resembles that of the vertebrate eye. 
The true Lamellibranchiata have lost their head, and 
with it both their cephalic eyes. In lieu of these, many 
Lamellibranchs (e.g. Pecten) have numerous beautiful 
green eyes developed on the margin of the mantle, i.e. 
on the external border of a great dorsal fold of the 
integument, that invests the body like a mantle. In 
the higher Vermes we meet generally with a pair of 
eyes on the head. But individual Annelida (Fabricia) 
have in addition a pair of eyes placed posteriorly in the 
tail, and others (Polyophthalmus) have a pair of eyes on 
every limb. These and many similar facts prove most 
clearly that the eyes, like other organs of the body, 
have built themselves up by adaptation to external 

The marvellous force that adaptation has constantly 
exerted on the advancing perfection of the highest 



organs of sense, can be followed out in detail during 
the short time of the history of human civilisation up 
to the present day. Especially does the higher color- 
sense seem to be incomparably more developed in us 
at the present day than in our ancestors, thousands of 
years before. Many observers have already come to 
the conclusion that the human race, two thousand years 
ago, only distinguished the lower colors of the spectrum, 
red, orange, yellow, whilst the higher tones, the green, 
blue, violet colors, were then unknown to man. Many 
strong evidences in favor of this idea are forthcoming 
in the works of art, and the classical writings of ancient 
time ; but, on the other hand, it must be confessed that 
many facts tell against this view. The contest on this 
point in which the English statesman, Gladstone, the 
Breslau ophthalmist, Magnus, and also Dr. E. Krause, 
among others, have taken part, is still in full force to- 
day. When we consider how exceedingly variable is 
the development of the sense of color at the present 
hour among civilised races, and in different individuals, 
how widely prevalent in various classes of the commu- 
nity is color-blindness or Daltonism, we ought, at least, 
to regard it as certain that the highly developed sense 
of color of the present time, is a recent production of 
civilised Evolution. The recent development of land- 
scape-painting speaks powerfully in support of this 
view. This branch of art has, of late, in our own 
century grown to a perfection undreamed of before. 
We perceive the more delicate beauties of natural 
coloring incomparably more clearly than our mediaeval 
forefathers. The delicate cones of the retina that are 
concerned in this more cultured color-sense, have, there- 
fore in all probability, gradually advanced in develop- 
ment during the last ten centuries. Even now-a-days 
we see in the surviving savage races a crudity as to this 
sense, and as to the sense of tone, that shocks our cul- 
tured sense of beauty. Our little ones also, like the 
savages, love collocations of glaring colors that grate 
upon us, and susceptibility to the harmony of delicate 
shades of color is the latest product of aesthetic educa- 

In the eye, also, as in the ear, it is education and 



improvement, use and habit, in a word adaptation, that 
have gradually raised the sense-organ and its aesthetic 
capabilities to such a height, and by heredity this mag- 
nificent heirloom is handed on from generation to 
generation. Seeing the astonishing advance already 
made by our senses of color and of tone in historic 
time, we may dare to hope that these will rise by 
yet further advances, and under careful training, to a 
far higher condition of perfection. And when we think 
that art, most noble, most regal of humanity's posses- 
sions, depends for its first principles upon the advance 
of these two aesthetic sense-organs, we may hope to 
make more and more perfect the arts of music and of 
painting in the coming years, through the ever-growing 
perfection of ear and eye. Thus the study of Evolution 
at this hour, in its application to the development of 
the sense-organs, gives us an insight, full of inspiration 
to happiness, into the more perfect world that is to be. 


E X. 

Aborigines . 
Acalephse . 
Accipenser . 
Actinozoa . 
Adaptation . 

Alcippe, Eye of 
Alciopidae ... 
American ... 



, 281 
82, 84 

71, 72, 83, 84 
79, 84, 85, 86 



58, 76, 

70, 72, 80, 81, 

75, 76, 170, 221, 247, 
249, 281, 3L0 
Amphibia ... 58, 62, 77, 86, 208, 
281, 307, 331, 343 
Amphioxus ... 58, 65, 74, 76, 78, 
8G, 247, 252, 326 
Amphirrhini ... 59, 86 

Anamnia ... ... ... 86 

Anatomy ... ... ... 195 

— (Comparative Evi- 
dence of as to Evolution) 

266, 267, 268, 269 

Antimeres ... 
Ants, Sense of duty in 



Archebiosis ... 
Arthropoda . . 
Articulata ... 
Artificial Selection 


77, 86 
84, 86 

... 33, 46, 57 
137, 189, 193 
40, 57, 121, 343 
325, 326 
18, 45 
74, 76, 78 

58, 74, 75, 76, 324 


Atoms 231 

Souls 231, 232 

Auditory cells of dove ... 321 

Auerbach 222, 236 

Aurelia aurita ... ... 101 

Australian negroes 71, 81, 82, 
85, 86, 99, 333 
Aves... 58, 63, 86, 281, 307, 343 

Bathybius ... 
Batrachian ... 


Bessels, Von 
Blastoderm ... 
Boniface VIII., Pope 


Bushman . . . 

... 84 
33, 224 
... 62 
... 221 
... 156 
... 156 
... 15 
... 224 
... 196 
64, 69, 70 
128, 146 
81, 82 
... 194 
... 324 
79, 86 




145, 219, 286 




Calcareous sponge 194 

Campanularia Johnstoni . . . 106 

Camper 1 94 

Canidse 37 

Carbon (physical basis of life) 229 

Carbon Theory 229 

Carboniferous 64 

Carmarina hastata... ... 100 

Carnivora 38, 68, 86 

Catarrhini ... 49, 71, 72, 73, 77 

Cats 16, 37 

Cells 121 

Cell-souls ... 133,135,295 
Cell Theory... 119,121,196 

Cenogenesis 294 

Ceratodus 61 

Cetacea ... 38, 64, 67, 86 




Cheiroptera... 48, 69, 70, 71, 86 
Chimpanzee ... ... 86 

Chiromys ... 70 

Chorda dorsalis ... 40,58 
Chorology ... 200, 201, 294 

Cienkowski 223 

Cilia 141 

Ciliata ... 76,169,300,312 
Classes (Vertebrate) ■ ... 86 
Classification (genealogical) 193 
Clover ... ... ... 16 

Coeciliae ... ... ... 62 

Ccelenterata ... 41, 335 

Cohn 221 

Colobus 77 

Coloring ... ... ... 17 

Communities ... ... 122 

Comparative Anatomy ... 48 
Comparative Philology ... 48 
Consciousness ... ... 150 

Conservation of Energy ... xi. 
Copernicus ... 29, 30, 35, 52 
Coreo- Japanese ... ... 83 

Corymorpha ... ... 163 

Cosmogonies ... ... 5 

Cosmos ... ... ... xii. 

Craniata ... ... 58,86 

Craspedote Medusa ... 104 
Cretaceous ... ... 60, 63, 99 

Crinoideai ... ... ... 41 

Crustacea ... 121, 244, 324, 343 
Crystallography ... ... 294 

Cuckoo ... ... ... 156 

Cuvier 6,7,194 

Cycla% Auditory vesicle of 324 
Cyclostoma ... ... ... 86 

Cytoblast 222 

Cytods - 122 

Cytula 245 


Daltonism 345 

Daphnidae 244 

_ Darwin ... 97, 104, 169, 198, 

200, 211, 212, 213, 
214, 215, 216, 217, 
218, 238, 246, 253, 
254, 277, 278, 279, 
281, 293, 343 
Deccan ... ... ... 84 

Democritus 93, 287 

Dermo-muscular layer ... 144 
Design ... ... ... 34 

Desmidiacese ... ... 242 

Development of auditory 
vesicle ... ... ... 328 


Devonian ... ... ... 60 

Diatomaceao ... ... 242 

Diatoms ... ... ... 221 

Didelphia ... ... ... 66 

Dinosauria ... ... ... 63 

Dionoea . . ... ... 172 

Dipneusta 58 

Dipnoi 61, 77, 86 

Discoplacental ... ... 70 

Discoplacentalia 48, 67, 70, 71, 86 

Dogs 157 

Dolichocephali ... 79,86 

Dongolesi ... 84 

Dravidians ... ... ... 84 

Du-Bois Raymond . . . 150, 158 
Dysteleology ... ... 294 


Echinida ... ... ... 41 

Echinodermata ... 41,57 

Ectoderm 128, 146 

Edentata 67, 76 

Ehrenberg 170 

Elsberg 254,255,256 

Embryology ... 50, 197 

Endoderm 129, 146 

Equivocal Goneration ... 33 

Eskimos 80, 83, 98 

Eucope 107,161,322 

Europeans ... ... ... 79 

"Evidences as to man's place 

in Nature ... ... 70 

-Evolution 98, 34G 

Present position of, in 

relation to Science 277-292 
Theory of ... 289, 300 


Fabricia ... ... ... 344 

Fechner 172 

Felidse 37 

Felis 37 

Field-mice ... ... ... 16 

Final Causee ... ... 34 

Finns 83 

Firola 323 

Fishes 60 

Fission of the Ovum ... 127 

Flagellata 310 

Flowering Plants 120 

^orskalia Ill 

Frog 62 

Fulati 84 


Galenus-Claudius ... ... 194 

Galeopithecus ... ... 70 




Galileo 30, 31, 35 

Ganglion-cells ... 126, 150 
Ganoidei ... ... ... 60 

Gasteropoda 344 

Gastraea 247, 281 

Gastrseada ... 75, 76, 241, 315 
Gastrula, 76, 128, 146,281,313, 

Gauchos 98 

Gegenbauer 214, 285, 295 

General Morphology, 74, 212, 214, 

Genus 7 

Geology 295 

Germinal Vesicle 127 

Geryonidse 100 

Gladstone 345 

Goethe, xi., 10, 42, 46, 97, 155, 

231, 248, 252, 278 

Gorilla 78 

Gregarina 225 

Gregarinida ... ... 242 

Gustatory Cells 318 


Hag 59 

Heredity ... 11,44,98,236 

Hering, Ewald 251 

Hertwig 222, 236 

Himatega 76 

Hippopodius ... ... 116 

History 22 

Holothurida 41 

Homines ... ... ... 86 

Homo 79 

afer 81 

albus 79 

americanus 83 

arcticus ... ...83,86 

australis 82 

cafer ... ... ... 82 

dravida 84 

fuscus ... ... 79 

hottentotus 81 

luteus 79 

mediterraneus ... 84 

mongolus ... ... 83 

niger 79 

nuba ... ... ... 84 

papua... ... ... 81 

polynesius ... ...82,86 

primigenius ... ...80,81 

rufus 79 

sapiens ... ... 78 

Homologies... ... ... 11 

Hooker 4, 204 


Hottentots ... 78, 81, 82, 85, 86 

Huber 97, 155 

Huyghens ... ... ... 257 

Human ear ... ... ... 332 

Humble-bees ... ... 16 

Humboldt 200 

Hungarians... ... ... 83 

Huschke, Emil . . . 302, 328 
' Huxley 4, 33, 35, 49, 70, 71, 
72, 204, 222, 285 
Hydra, Divisibility of ... 161 
Hydrae ... 158, 159, 315 

Hydroid polyps ... ... 102 

Hydromedusaa 102 

Hydrozoa 41, 111, 241 

Hylobates ... 72 

Hyperboreans ... ... 83 


Indo-Chinese 83 

Indo-Europeans ... ... 84 

Indri 70 

Infusoria ... 170, 171, 242 

Insecta 121, 153 

Insectivora ... 48, 69, 70, 71, 86 
Instinct ... 97, 155, 156, 157 
Intermaxillary bone ... 47 
Invertebrata .. 75, 247 
J . 

Jurassic 60 


Kangaroo ... 66 

Kant xi, 137, 215, 245, 278, 293 

Kepler 30 

Kleinenberg ... 124, 160 

Kosmos 225 

Kowalewski ... ... 74 

Krause 345 


Labyrinthodonta 62 

Labyrinthula .. ... 76 

Lamarck xi., 9, 39, 42, 43, 46, 
52, 197, 198, 237, 
257, 278, 293 
^Lamarckism ... . . 44 

Lamellibranchiata 344 

Lampreys ... 59 

Lancelot ... ... ... 58 

Latreille ... ... ... 97 

Leuwenhoeck 196 

Leibnitz 26, 32, 287 

Lemur ... ... 70, 97 

Lemuria 73, 80 

Length, geological epochs. . . 20 

Lepidosiren 61 

Lepidosteus 60 



Lichanobus . . 


Lissotrichi . 
Lori ... 







... 7, 190, 284 



70, 71 
4, 204, 246, 293 


Madonna ... ... ... 138 

Magnus ... ... ... 345 

Maki 70 

Malacca ... ... ... 82 

Malay 79, 81, 82, 83, 84, 85, 86 

Malleus 38 

Malpighi ... ... 195, 196 

Mammalia .. 58, 63, 77, 86, 104, 
307, 332, 343 

Mammalian Orders ». 86 

Man's place in Nature 70, 104 

Marsupialia 38, 86 

Marsupials ... 4, 21, 64, 66, 77 

Martins 77 

Medusa, 100, 101, 161, 162, 165, 

167, 244, 323, 325 
Memory of plastidules ... 234 

Men-apes 77 

Menocerca ... ... ••• 77 

Mesocephali 79 

Metagenesis ... ... 243 

Metameres ... ... ... 121 

Metastasis ... ... ... 160 

Metazoa ... ... ••• 313 

Miocene age ... ... 73 

Mollusca ... 39,57,74,344 

Monas 223 

Monera, 33, 44, 57, 75, 76, 78, 122, 
223, 224, 225, 226, 234, 242, 

247, 249, 256, 286 

Monerula 226, 256 

Mongols (Asiatic), 79, 83, 84, 85, 

Monism ••• 287 

Monistic Philosophy ... 277 

Monodelphia 66 

Monorrhini ... ... 58, 77, 86 

Monotremata ... 64, 65, 77, 86 
Morphology ... ••• 295 

Morula ... ... 128,144 

Muller ... 214,309,333 
Multicellular Organisms ... 76 
Musea, Proboscis of ... 317 
MustclidsQ ... 38 

Myxinoids ... 



... 257 
... 326 
... 59 
... 77 
41, 242 


Natural History of Creation 
Natiirliche Schopfungsgech- 

chiste 97 

--Natural Selection, xi, 14, 44, 73, 
85, 198, 211, 241, 278, 300, 344 
--Natural System ... ... 37 

Nectocalyces ... ... 166 

Nematoidea... ... ... 41 

Nervous System of Man ... 140 

of Turbellaria ... 141 

Neuro-muscular cells 160, 162 
Neuroptefa ... ... ... 153 

Newton t.. 25,31,52,257 

Notochord ... * 40 

No vara Expedition... ... 85 

Nysterida ... 69 


Oken ... 10, 42, 278,293 

Olfactory cells 320 

Olynthus ... 148, 171, 312, 314 
Onchidium ... ... ... 344 

Ontogeny, 118, 129, 197, 199, 212, 
269, 270, 271, 272 
Opossum ... ... ... 66 

Orang greater ... ... 86 

lesser... ... ... 86 

Orders, The fourteen Mam- 
malian ... ... ... 86 

Organs 121, 138 

Organ of Oorti ... 327,329 
.Origin of Human Race ... 27 
of Species ...10,32,43,211 

Osseous fishes 
Otolicnus ... 

cells ... 


153, 326 
.... 60 
... 70 
... 126 
... 172 
... 243 

Pachycardia ... ... 58 

Pachydermata ... ... 68 

Palaeontology 51 

Evidence of, as to 

Evolution ...264, 265, 266 

Pallas 58, 194 

Pander 197 

Pangenesis ... 216, 217, 252 




Papuan 78, 82, 85, 86, 99, 333 

Pecten 344 

Pedigree of man 55 

Perennibranchiata ... ... 62 

Perigenesis of the Plastidule 

207, 211, 216, 230, 249 
Petromyzon ... 59, 77 

Pfaundler 238 

Phallusia 74 

Pharyngobranchii 65 

Philology 48 

Philosophie Zoologique ... 43 
Phylogenesis ... ... 213 

Phylogeny 129, 197, 212, 262, 
263, 282, 295 

Phylum 39 

Physemaria 315 

Pigment layers of eye ... 341 


Placentalia ... 
Planaeada ... 
Plasm bodies 






Pliocene age 


Polynesia ... 
Polyophthalmus ... 
Polyp (Freshwater) 
Polypterus ... 
Pongines (African) 
Pongo Troglodytes... 
Primary age 
Primitive fishes 
Pritchard ... 
Proofs of Evolution 
Prorodon ... 

69, 86 
58, 77, 86, 282, 
307, 331, 343 



Protamnia .., 
Protamceba .. 
Protamyxa .. 

... 229 
... 230 
121, 122 
... 230 
... 65 
.. 73 
... 166 
... 194 
... 82 
... 344 
158, 323 
... 60 
... 86 
72, 86 
... 50 
... 77 
79, 81 
74, 75 
... 77 
259, 261 
169, 312 
48, 69, 70, 71, 73, 
77, 86 




62,' 77, 320 
122, 170, 171, 223, 


Protophy ta 

Theory ... 

Psychology .. 
Pterocyna . . 


240, 242 
41, 222 
.. 221 
.. 61 
41, 170, 242, 311 


136, 202 



Quadrate bone 


38, 64, 69, 71 


Radiata 41 

Raj 89 60 

Ray 60 

Religion, Natural ... ... 291 

Reproduction ... ... 241 

Reptilia 58, 62, 86, 281, 343 
Revolution of the Heavenly 

235, 240, 249, 311, 335 


... 30 

Rhizopoda ... 

41, 300 

Ritter, Carl... 

... 200 

Rodentia ... 48, 

69, 70, 71, 86 


... 139 


... 68 



... 100 



Satyrii (Asiatic) . 

... 86 

Satyr us 

... 72 

moris ... 


orang ... 





... 195 

Schelling ... 

... 278 



Schleicher ... 

... xiii. 

Schleiden, 145, 196 

219, 257, 28§ 





Schwann ... 

.145, 196, 219 








127, 143 



Semitic peoples 







Sense Organs, Origin and 

Development of ... 299, 300 

Sepia 344 

Sharks 60 

Silurian 50, 59 

Simise 70, 71, 86 

Siphonophora 109, 118, 119, 
120, 165, 245 

Slade 138 

Snails 324 

Social instincts 297 

Solidungula 68 

Soul 135 

cells ... 126, 133, 135 

functions 172 

■ life 138 

Evolution of ... 285 

South Sea 82 

islands ... ... 98 

Sozobranchii ... ... 77 

Sozura 77 

Spallanzani 308 

Sparsiplacentalia ... ... 69 

Speech ... 22 

Species 79 

Spencer ... ... ... 204 

Spinoza 131, 287 

Spiritualism ... ... 137 

Spongida ... ... ... 241 

Spontaneous Generation ... 33 
Squali ... ... ... 60 

Steenstrupia ... 163,164 

Stenops 70, 77 

Sterlet 60 

Sterne Carus ... ... xii 

St. Hilaire, G. ... 10, 42, 278 

Strasburger 222 

" Strobila 104 

Struggle for Life ... 14, 45 

Sturgeon ... 60 

Sunda Islands ... ... 70 

Sympathetic Coloring ... 17 

Synamceba 76 

Systematic resume of the 
eight Vertebrate classes 86 


Tamiada ... ... ... 41 

Tailed Apes 77 

Tarsius 70 


Tasmania ... 81 

Teleostei 60 

Termites ... 154 

Tertiary age ... ... 51 

Thallus 121 

Thread-cell 1 14 

Thrissopidae 61 

Toad 62 

Torpedo 126, 150 

Transformism ... ... xi 

Trematode ... ... ... 41 

Triassic ... ... 45, 63 

Triton 77 

Tunicata 41, 74 

Turbellaria ... 75, 76, 139, 302 

Ulotrichi 86 

Ungulata 38, 67, 86 

Ural 83 

Ursidae 38 


Vampyrella 33, 223 

Van Dieman's Land ... 81 

Variability 44 

Variation 12 

Vermes, 41, 57, 74, 75, 76, 281, 
302, 307, 315, 323, 335, 338 
Vertebrata, 39, 247,316,326, 331, 

338, 339, 342, 343, 344 

auditory vesicles of 327 

Vertebrate Glasses... ... 86 

Villiplacentalia 67, 86 

Virchow, 156, 169, 196, 219, 286 

Vital Force 34, 201 

Vivisection... 201 

Vogt (Carl) 33,97 

Von Bar 39, 207 


Weisbach 85 

White ants ... ... ... 154 

Wolff 197 


Zonoplacentalia ... ...67,86 

Zoologie Philosophique ... 9 
Zoology, Progress and Work 

of 185 

Plan of Chief Divi- 
sions of Scientific ... 207 
Zoophyta ... ... ... 57 

Zootomy 11*5 

My thanks are due to my friend Captain Bingham for the 
preparation of the Index. — E. B. A. 

5450 31